Update phone number hook documentation

[sahilsuman933]

Description

• Documented support for call.ending hooks on phone numbers.
• Added an example demonstrating how to transfer calls to a fallback number when assistant requests fail (e.g., assistant-request-failed, assistant-request-returned-error).
• Updated usage details, filter restrictions, and enablement steps for phone number hooks.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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[github-actions]

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