SIP INVITE Voicegain from Twilio, SignalWire, Telnyx CPaaS

Voicegain Telephony Bot API allows developers to use Voicegain Speech-to-Text to build Voice Bots or programmable speech IVR using a simple callback API. With latest Voicegain Platform release 1.21.0 it is now possible to establish SIP sessions to Voicegain Telephony Bot API using a simple SIP Invite.

Before release 1.21.0, the only way for voice app developers to use the Voicegain Telephony Bot API was to call the application using phone numbers that were purchased from Voicegain (via the Web Console). However, we have always wanted to allow clients to bring their own carrier or CPaaS platform and this release allows developers to do just that.

At Voicegain our focus is on offering our ASR/Speech Recognition functionality and our full featured Speech-to-Text APIs. We understand developers rely on their CPaaS platforms for a whole host of important features - messaging, emails, conferencing and international coverage. Now, it is possible to integrate Voicegain Telephony Bot API with any CPaaS that supports SIP Invite. You can combine powerful and affordable Speech Recognition features of the Voicegain Platform with the comprehensive  API features of these CPaaS platforms

We have already tested SIP Invite extensively on Twilio, SignalWire, and Telnyx platforms. Other similar platforms should also work without issues. We will report any additional platforms that we have explicitly tested in the future.

How SIP INVITE works with Twilio & SignalWire

On Twilio and SignalWire platforms is trivial to establish SIP session to Voicegain. The only thing needed is the <Dial><Sip> command from TwiML or LaML, for example:

Some notes about the above example:

  • The SIP URI user name is a unique random identifier assigned on Voicegain Platform to each Telephony Bot Application.
  • After the SIP connection gets established, the application prompts and speech recognition will be under control of Voicegain Platform based on commands passed using our Telephony Bot API
  • Once Voicegain `disconnect` command is issued, the control of the application flow will be returned back to the host platform (i.e. Twilio, SignalWire or any other CPaaS platform).
  • It is possible to pass custom headers to Voicegain during SIP Invite - this way it is possible to associate host sessions with Voicegain sessions.
  • It is possible to make multiple <Dial><Sip> requests to Voicegain from host application during a single host session.

On our github you can find sample code showing how to dial a outbound call and then bridge it to Voicegain SIP:

What about Telnyx

On Telnyx we tested SIP INVITE using the Telnyx Call Control API. The only functional difference from Twilio and SIgnalWire is that on Telnyx you cannot choose TCP as SIP transport (only UDP is supported).

Here is a sample Python code showing how to dial Voicegain SIP:

The complete code for an AWS Lambda function that dials a number using Telnyx and then bridges it to Voicegain SIP is available here: platform/ at master · voicegain/platform (

What can I build with the Telephony Bot API?

Our Telephony Bot API is a callback API in similar fashion as TwiML or LaML. The main difference is that it is based on JSON and our functionality is focused on Speech Recognition. You can read more about it in our blog post announcing release of that API back in August.

On out Github you can find an example of a Node.js function on AWS Lambda that demonstrates how to interface Voicegain Telephony Bot API with a RASA NLU bot: platform/examples/voicebot-lambda-vg-rasa at master · voicegain/platform (

You can also check out our sample python function code on AWS Lambda which shows how to implement more traditional (VoiceXML like) IVRs with the use of Speech grammars on top of our Telephony Bot API: platform/declarative-ivr at master · voicegain/platform (

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