docs(mobile-phone-waterfall): add FAQ for diagnosing inaccurate phone numbers

docs/mobile-phone-waterfall.md

@@ -97,3 +97,8 @@ ClearoutPhone is available for every optimization option and runs on your own Cl
 
 **Will stricter validation cost more credits?**
 It can. Rejecting more numbers means the waterfall calls more providers per contact before it finds an acceptable result, which can increase spend and reduce fill rate — in exchange for higher-quality output.
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+**Some of the phone numbers the waterfall is returning are wrong or outdated — how do I find out which provider is responsible?**
+Enable **Output name of successful provider?** in your waterfall's **Additional column settings**. This adds a column to your table showing which provider returned the accepted number for each row. When reps flag a bad number, check that column to identify a pattern — if the same provider appears repeatedly on flagged rows, that provider may have stale data for your specific audience or geography.
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+To compare providers directly before committing to a waterfall order, run individual provider enrichments on the same set of contacts rather than nesting waterfalls. Running them as separate columns makes each provider's result visible and attributable, so you can evaluate accuracy across sources side by side. Once you've identified which providers consistently return the most accurate results for your use case, reorder or disable underperformers in the waterfall's **Waterfall sequence** configuration.