diff --git a/lib/Command/PhoneNumber/AddPhoneNumber.php b/lib/Command/PhoneNumber/AddPhoneNumber.php
index e65e76259a9..8785684ecb3 100644
--- a/lib/Command/PhoneNumber/AddPhoneNumber.php
+++ b/lib/Command/PhoneNumber/AddPhoneNumber.php
@@ -11,8 +11,8 @@
 use OC\Core\Command\Base;
 use OCA\Talk\Model\PhoneNumber;
 use OCA\Talk\Model\PhoneNumberMapper;
+use OCA\Talk\Service\PhoneNumberValidation;
 use OCP\AppFramework\Db\DoesNotExistException;
-use OCP\IPhoneNumberUtil;
 use OCP\IUser;
 use OCP\IUserManager;
 use Symfony\Component\Console\Input\InputArgument;
@@ -24,7 +24,7 @@ class AddPhoneNumber extends Base {
 
 	public function __construct(
 		private IUserManager $userManager,
-		private IPhoneNumberUtil $phoneNumberUtil,
+		private PhoneNumberValidation $phoneNumberValidation,
 		private PhoneNumberMapper $mapper,
 	) {
 		parent::__construct();
@@ -66,12 +66,12 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 		}
 		$userId = $user->getUID();
 
-		$phoneNumberStandard = preg_match('/^[0-9]{1,20}$/', $phoneNumber) ? $phoneNumber : $this->phoneNumberUtil->convertToStandardFormat($phoneNumber);
-		if ($phoneNumberStandard === null) {
+		try {
+			$phoneNumber = $this->phoneNumberValidation->validateNumber($phoneNumber);
+		} catch (\InvalidArgumentException) {
 			$output->writeln('<error>Not a valid phone number ' . $phoneNumber . '. The format is invalid.</error>');
 			return self::FAILURE;
 		}
-		$phoneNumber = $phoneNumberStandard;
 
 		try {
 			$entry = $this->mapper->findByPhoneNumber($phoneNumber);
diff --git a/lib/Command/PhoneNumber/ImportPhoneNumbers.php b/lib/Command/PhoneNumber/ImportPhoneNumbers.php
index ac9d4098426..32c46978369 100644
--- a/lib/Command/PhoneNumber/ImportPhoneNumbers.php
+++ b/lib/Command/PhoneNumber/ImportPhoneNumbers.php
@@ -11,8 +11,8 @@
 use OC\Core\Command\Base;
 use OCA\Talk\Model\PhoneNumber;
 use OCA\Talk\Model\PhoneNumberMapper;
+use OCA\Talk\Service\PhoneNumberValidation;
 use OCP\IDBConnection;
-use OCP\IPhoneNumberUtil;
 use OCP\IUser;
 use OCP\IUserManager;
 use Symfony\Component\Console\Input\InputInterface;
@@ -23,7 +23,7 @@ class ImportPhoneNumbers extends Base {
 
 	public function __construct(
 		private IUserManager $userManager,
-		private IPhoneNumberUtil $phoneNumberUtil,
+		private PhoneNumberValidation $phoneNumberValidation,
 		private PhoneNumberMapper $mapper,
 		private IDBConnection $db,
 	) {
@@ -72,12 +72,12 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 				continue;
 			}
 
-			$phoneNumberStandard = preg_match('/^[0-9]{1,20}$/', $row[0]) ? $row[0] : $this->phoneNumberUtil->convertToStandardFormat($row[0]);
-			if ($phoneNumberStandard === null) {
+			try {
+				$row[0] = $this->phoneNumberValidation->validateNumber($row[0]);
+			} catch (\InvalidArgumentException) {
 				$output->writeln('<error>Not a valid phone number ' . $row[0] . '. The format is invalid.</error>');
 				return self::FAILURE;
 			}
-			$row[0] = $phoneNumberStandard;
 
 			$user = $this->userManager->get($row[1]);
 			if (!$user instanceof IUser) {
diff --git a/lib/Service/PhoneNumberValidation.php b/lib/Service/PhoneNumberValidation.php
new file mode 100644
index 00000000000..69fb017cb5f
--- /dev/null
+++ b/lib/Service/PhoneNumberValidation.php
@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Talk\Service;
+
+use OCP\IConfig;
+use OCP\IPhoneNumberUtil;
+
+class PhoneNumberValidation {
+
+	public function __construct(
+		protected IPhoneNumberUtil $phoneNumberUtil,
+		protected IConfig $config,
+	) {
+	}
+
+	/**
+	 * Validate input as a phone number
+	 *
+	 * - Local number: allow
+	 * - International number
+	 *      a. If valid, strip + and allow
+	 *      b. If invalid, throw
+	 * @throws \InvalidArgumentException When the number is invalid
+	 */
+	public function validateNumber(string $phoneNumber): string {
+
+		if (
+			// Not an internation number
+			!str_starts_with($phoneNumber, '0')
+			// And matches a local number or dial-through
+			&& preg_match('/^[0-9]{1,20}$/', $phoneNumber)
+		) {
+			return $phoneNumber;
+		}
+
+		$defaultRegion = $this->config->getSystemValueString('default_phone_region') ?: null;
+		$standardPhoneNumber = $this->phoneNumberUtil->convertToStandardFormat($phoneNumber, $defaultRegion);
+
+		if ($standardPhoneNumber === null) {
+			throw new \InvalidArgumentException();
+		}
+
+		if (str_starts_with($standardPhoneNumber, '+')) {
+			return substr($standardPhoneNumber, 1);
+		}
+
+		throw new \InvalidArgumentException();
+	}
+}
diff --git a/lib/SetupCheck/SIPConfiguration.php b/lib/SetupCheck/SIPConfiguration.php
index 0f2fb2a387b..5eed31f6b3e 100644
--- a/lib/SetupCheck/SIPConfiguration.php
+++ b/lib/SetupCheck/SIPConfiguration.php
@@ -9,6 +9,7 @@
 namespace OCA\Talk\SetupCheck;
 
 use OCA\Talk\Config;
+use OCP\IDBConnection;
 use OCP\IL10N;
 use OCP\SetupCheck\ISetupCheck;
 use OCP\SetupCheck\SetupResult;
@@ -16,6 +17,7 @@
 class SIPConfiguration implements ISetupCheck {
 	public function __construct(
 		protected readonly Config $talkConfig,
+		protected readonly IDBConnection $connection,
 		protected readonly IL10N $l,
 	) {
 	}
@@ -39,9 +41,35 @@ public function run(): SetupResult {
 		if ($this->talkConfig->getSignalingMode() === Config::SIGNALING_INTERNAL) {
 			return SetupResult::success($this->l->t('Using the SIP functionality requires a High-performance backend.'));
 		}
+
+		$query = $this->connection->getQueryBuilder();
+		$query->select('phone_number')
+			->from('talk_phone_numbers')
+			->where($query->expr()->like('phone_number', $query->createNamedParameter(
+				$this->connection->escapeLikeParameter('+') . '%'
+			)))
+			->orWhere($query->expr()->like('phone_number', $query->createNamedParameter(
+				$this->connection->escapeLikeParameter('0') . '%'
+			)));
+
+		$result = $query->executeQuery();
+		$invalidNumbers = $result->fetchFirstColumn();
+		$result->closeCursor();
+
+		if (!empty($invalidNumbers)) {
+			$message = $this->l->t("Assigned Talk phone numbers must not start with + or 0. Please remove or update the following numbers:\n{list}");
+			$message = str_replace(
+				'{list}',
+				implode("\n", $invalidNumbers),
+				$message
+			);
+			return SetupResult::error($message, 'https://portal.nextcloud.com/article/Nextcloud-Talk/Nextcloud-Talk-Phone/Direct-Dial-in#content-provisioning');
+		}
+
 		if ($this->talkConfig->getSIPSharedSecret() === '' && $this->talkConfig->getDialInInfo() === '') {
 			return SetupResult::info($this->l->t('No SIP backend configured'));
 		}
+
 		return SetupResult::success();
 	}
 }
diff --git a/tests/integration/features/command/phone-number.feature b/tests/integration/features/command/phone-number.feature
index 0723034f001..af7c52fdd8f 100644
--- a/tests/integration/features/command/phone-number.feature
+++ b/tests/integration/features/command/phone-number.feature
@@ -14,19 +14,19 @@ Feature: command/phone-number
 
     Given invoking occ with "talk:phone-number:add +49-160-123-12-12 participant1"
     Then the command failed with exit code 0
-    And the command output contains the text "Phone number +491601231212 is now assigned to participant1"
+    And the command output contains the text "Phone number 491601231212 is now assigned to participant1"
 
-    Given invoking occ with "talk:phone-number:find --phone +491601231212"
+    Given invoking occ with "talk:phone-number:find --phone 491601231212"
     Then the command failed with exit code 0
-    And the command output contains the text "Phone number +491601231212 is assigned to participant1"
+    And the command output contains the text "Phone number 491601231212 is assigned to participant1"
 
-    Given invoking occ with "talk:phone-number:find --phone +49160123121234"
+    Given invoking occ with "talk:phone-number:find --phone 49160123121234"
     Then the command failed with exit code 1
-    And the command output contains the text "Phone number +49160123121234 could not be found"
+    And the command output contains the text "Phone number 49160123121234 could not be found"
 
     Given invoking occ with "talk:phone-number:find --user participant1"
     Then the command failed with exit code 0
-    And the command output contains the text "participant1 has phone number +491601231212 assigned"
+    And the command output contains the text "participant1 has phone number 491601231212 assigned"
 
     Given invoking occ with "talk:phone-number:find --user participant2"
     Then the command failed with exit code 1
@@ -34,13 +34,13 @@ Feature: command/phone-number
 
     Given invoking occ with "talk:phone-number:add +49-160-123-1213 participant1"
     Then the command failed with exit code 0
-    And the command output contains the text "Phone number +491601231213 is now assigned to participant1"
+    And the command output contains the text "Phone number 491601231213 is now assigned to participant1"
 
     Given invoking occ with "talk:phone-number:find --user participant1"
     Then the command failed with exit code 0
     And the command output contains the text "participant1 has the following phone numbers assigned:"
-    And the command output contains the text "- +491601231212"
-    And the command output contains the text "- +491601231213"
+    And the command output contains the text "- 491601231212"
+    And the command output contains the text "- 491601231213"
 
     Given invoking occ with "talk:phone-number:add +49-160-123-1212 participant2"
     Then the command failed with exit code 1
@@ -48,7 +48,7 @@ Feature: command/phone-number
 
     Given invoking occ with "talk:phone-number:add --force '+49-160-123-12-12' participant2"
     Then the command failed with exit code 0
-    And the command output contains the text "Phone number +491601231212 is now assigned to participant2"
+    And the command output contains the text "Phone number 491601231212 is now assigned to participant2"
     And the command output contains the text "Was assigned to participant1"
 
     Given invoking occ with "talk:phone-number:add 23 participant2"
@@ -58,14 +58,14 @@ Feature: command/phone-number
     Given invoking occ with "talk:phone-number:find --user participant2"
     Then the command failed with exit code 0
     And the command output contains the text "participant2 has the following phone numbers assigned:"
-    And the command output contains the text "- +491601231212"
+    And the command output contains the text "- 491601231212"
     And the command output contains the text "- 23"
 
     Given invoking occ with "talk:phone-number:find --user participant1"
     Then the command failed with exit code 0
-    And the command output contains the text "participant1 has phone number +491601231213 assigned"
+    And the command output contains the text "participant1 has phone number 491601231213 assigned"
 
-    Given invoking occ with "talk:phone-number:remove +491601231213"
+    Given invoking occ with "talk:phone-number:remove 491601231213"
     Then the command failed with exit code 0
 
     Given invoking occ with "talk:phone-number:find --user participant1"
@@ -78,3 +78,52 @@ Feature: command/phone-number
     Given invoking occ with "talk:phone-number:find --user participant2"
     Then the command failed with exit code 1
     And the command output contains the text "No phone number found for participant2"
+
+  Scenario: Phone number validation
+    Given invoking occ with "config:system:set default_phone_region --value DE"
+    # Invalid phone number with +
+    Given invoking occ with "talk:phone-number:add +4911223344 participant1"
+    Then the command failed with exit code 1
+    And the command output contains the text "Not a valid phone number +4911223344. The format is invalid."
+
+    # Valid German number
+    Given invoking occ with "talk:phone-number:add 004971112347 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 4971112347 is now assigned to participant1"
+
+    Given invoking occ with "talk:phone-number:add +4971112346 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 4971112346 is now assigned to participant1"
+
+    Given invoking occ with "talk:phone-number:add 4971112345 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 4971112345 is now assigned to participant1"
+
+    Given invoking occ with "talk:phone-number:add 071112348 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 4971112348 is now assigned to participant1"
+
+    # 01122 is not a valid prefix in Germany
+    Given invoking occ with "talk:phone-number:add 011223344 participant1"
+    Then the command failed with exit code 1
+    And the command output contains the text "Not a valid phone number 011223344. The format is invalid."
+
+    # Local PBX
+    Given invoking occ with "talk:phone-number:add 3001 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 3001 is now assigned to participant1"
+
+    # Local number 030 Berlin, but 01 is too short
+    Given invoking occ with "talk:phone-number:add 03001 participant1"
+    Then the command failed with exit code 1
+    And the command output contains the text "Not a valid phone number 03001. The format is invalid."
+
+    # Invalid German number but seen as local PBX
+    Given invoking occ with "talk:phone-number:add 4911223344 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 4911223344 is now assigned to participant1"
+
+    # Valid US number
+    Given invoking occ with "talk:phone-number:add 00112345678901 participant1"
+    Then the command failed with exit code 0
+    And the command output contains the text "Phone number 12345678901 is now assigned to participant1"