docs: document the call.ending phone number hook and hook filters

[chiranjeet-vapi]

Description

• fern/phone-numbers/phone-number-hooks.mdx says hooks support only call.ringing. The API also accepts call.ending, and call.ringing accepts a caller-prefix filter; neither was documented.
• Documents call.ending: it runs only when the inbound assistant request fails, takes a single do action (transfer or say), and can filter on call.endedReason with the six assistant-request ended reasons. Names the resulting ended reasons (call.ending.hook-executed-transfer / -say, already listed on the ended-reasons page).
• Documents the startsWith filter on call.ringing (key: "number", prefixes without +).
• Clarifies that call.ringing takes a do array and the first action runs.

Evidence

• live API spec (https://api.vapi.ai/api-json, 2026-10-14):
  • PhoneNumberHookCallEnding.do is a single oneOf (TransferPhoneNumberHookAction, SayPhoneNumberHookAction); PhoneNumberHookCallRinging.do is an array.
  • PhoneNumberCallEndingHookFilter: type enum oneOf, key enum call.endedReason, oneOf items enum: the six assistant-request-* reasons listed in the page.
  • PhoneNumberCallRingingHookFilter: type enum startsWith, key enum number, "Do not include the + prefix."
• fern/calls/call-ended-reason.mdx already lists call.ending.hook-executed-say and call.ending.hook-executed-transfer.
• That call.ending fires only on assistant-request failure, and that the first do action runs for call.ringing: verified against the current API implementation.

Testing Steps

• fern check (pinned 5.151.1) passes locally
• Check the preview deployment for the changed page

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Commit: e6fdee2
Status: waiting for staging docs URL