Upload an audio file so it can be used in voice calls. Provide a public HTTPS link to the file; we fetch it, store it, and return an audio_id to use when making calls (POST /v2/voice). Only MP3 files are supported — the URL must point to an .mp3 file. Uploading the same file again returns the same audio_id — it is stored only once. Requires the SMS & Voice feature to be enabled for your account by the Clint team.