Implement Native e164.org ENUM Resolution for Outbound Call Routing

[thorsager]

DESCRIPTION

Summary

We need to implement native support for e164.org ENUM (Telephone Number Mapping) resolution within the outbound routing engine of our custom SIP-based PBX. This feature will allow the PBX to translate dialed E.164 phone numbers into Internet-routable SIP URIs via a fast DNS query, bypassing standard PSTN carrier networks and facilitating free end-to-end VoIP peering.

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Architectural Implementation Workflow

1. Normalization: The PBX dial-plan engine intercepts an outbound call and converts the target number into a fully-qualified E.164 string (e.g., +12025550143).
2. String Reversal & Formatting: - Strip the leading +.
   • Reverse the digit string.
   • Separate every digit with a dot.
   • Append the e164.org zone.
   • Example string: 3.4.1.0.5.5.5.2.0.2.1.e164.org
3. DNS Core Resolution: Perform an asynchronous DNS query for standard NAPTR (Naming Authority Pointer) records against the generated domain.
4. Regex Execution & Routing Hand-off:
   • If the DNS query yields a valid SIP NAPTR record, parse the application regex to construct the target SIP URI (e.g., sip:reception@sip.example.com).
   • Rewrite the outbound INVITE request destination to this SIP URI.
   • Dispatch the call entirely over IP (via standard SIP/TLS core routing).
5. Failover handling: If the domain returns NXDOMAIN, times out, or contains no usable SIP URIs, fall back immediately and transparently to the next priority wholesale PSTN gateway trunk in the dial plan.

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Configuration Specifications

Administrators must be able to manage this feature via the PBX management dashboard/API:

• Enable ENUM Lookup: Global Boolean switch.
• Registry Base Domain: Text field defaulting to e164.org (allowing flexibility to test other trees like e164.arpa).
• Lookup Timeout Multiplier: Timeout constraint in milliseconds (suggested default: 350ms) to safeguard Post-Dial Delay (PDD) from degraded DNS infrastructure.

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Relevant Technical Standards & References

1. Official RFC Specifications

• RFC 6116 - The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM): Defines the core operation of converting standard telephone numbers into URI pointers utilizing DNS infrastructure.
• RFC 3403 - Dynamic Delegation Discovery System (DDDS) Part Three: The National Numbering Authority Pointer (NAPTR) DNS Resource Record: Specifies the format and parsing mechanisms required to interpret the NAPTR database entries.
• RFC 3261 - SIP: Session Initiation Protocol: Governing standard for assembling and forwarding the resulting IP-to-IP session invite requests.

2. Ecosystem Documentation

• e164.org Architecture Registry: The public community repository engine mapping E.164 paths natively to public Voice-over-IP endpoints.
• RFC 3761 (Obsolete but relevant): Legacy framework definition often referenced by older open-source VoIP components (e.g., early Asterisk/ResIP_Nodes modules) utilizing ENUM logic.

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Definition of Done (DoD)

• Core DNS utility successfully transforms E.164 formatted extensions into fully compliant reversed DNS query zones.
• Intercept component queries NAPTR structures and successfully processes regex syntax maps to deliver structural SIP targets.
• Dial-plan failover gracefully catches network latency or non-existent DNS targets, instantly redirecting the call to default SIP carrier trunks without session termination.
• Integration tests verify outbound SIP INVITE targets the matched domain rather than the default PSTN gateway trunk.