Vapi is the Voice AI platform for developers.

Vapi AI

Here is an example. This works. You can put it in Postman or anywhere. 

{
  "phoneNumberId": "6ee4a32d-5e94-4918-9445-7ca3181569d5",
  "customer": {
    "name": "Jeff",
    "number": "+{YourNumber}"
  },
  "assistant": {
    "transcriber": {
      "provider": "deepgram",
      "model": "general",
      "language": "en-US"
    },
    "model": {
      "provider": "openai",
      "model": "gpt-4",
      "temperature": 0.7,
      "maxTokens": 1000
    },
    "voice": {
      "provider": "azure",
      "voiceId": "en-US-JennyNeural",
      "speed": 1.0
    },
    "forwardingPhoneNumbers": [
      {
        "number": "+12135551234",
        "message": "This is the first forwarding number."
      },
      {
        "number": "+14155552678",
        "message": "This is the second forwarding number."
      },
      {
        "number": "+16175553456",
        "message": "This is the third forwarding number."
      },
      {
        "number": "+17185559876",
        "message": "This is the fourth forwarding number."
      },
      {
        "number": "+13105550789",
        "message": "This is the fifth forwarding number."
      }
    ]
  }
}