Cannot connect a phone number from a Telnyx sub-account.

Telnyx is connected to VAPI using an integration API key (not SIP).

When attempting to add a phone number from a sub-account to VAPI, a 404 error occurs.

This happens because VAPI is connected to the main Telnyx account, not the sub-account.

Proposed solution:

Similar to Twilio, allow specifying a telnyx_api_key instead of credentialId when adding Telnyx phone numbers.

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Without this change, Telnyx sub-accounts are unusable in VAPI, despite being a key feature for enterprise setups.