Reset Account Voice

Reset account voice to the system default.

Removes the account-level voice override so all AI agents fall back to the system default voice during phone calls (unless they have their own voice_id configured), starting with their next call. The previous value is not kept: read it with get_account_voice first if it may be needed again.

Returns account_id, default_voice_id null, resolved_voice_id (the system default) and a message.