SIP Forwarding

If you wish to foward an existing call to a SIP address, this is possible through Twilio's TwiML Dial SIP feature.

Create a Custom action step and name it twiml.

Set the Action Body to "Text", and you can Dial to a SIP address.

Example TwiML code:

<?xml version="1.0" encoding="UTF-8"?>
<Response>
    <Dial>
        <Sip>sip:alice@example.com;region=ie1</Sip>
    </Dial>
</Response>

Reference Twilio documentation for further information on how to format SIP addresses:
https://www.twilio.com/docs/voice/twiml/sip

SIP Outbound

It is possible to bind a Voiceflow agent to a SIP address via Twilio. For more information reference:
https://www.twilio.com/docs/voice/api/sending-sip
https://www.twilio.com/docs/voice/tutorials/how-to-route-calls-to-your-sip-network

On Twilio, under "Voice" > "Manage", you'll find a tab called "SIP domains". You'll need to register a new domain with Twilio and set it up.

Create a new credential, remember the username and password, this is what will be needed to log in.

Add your credential to "Voice Authentication".

Enable SIP Registration, and ensure your credential is added to "SIP Registration Authentication".

When you assign a Voiceflow agent to a phone number, it'll automatically populate the webhook with a URL.

Copy and paste this webhook into "Call Control Configuration". When a call is initiated from this SIP address, it will now hit the same webhook that the phone call hits.

To test this SIP functionality, you can download a free softphone service such as Zopier5 and register the login:
https://www.zoiper.com/

Create a new contact, just call it whatever you want. The "Phone" field can just be "test".
When you try to call outbound to that number it will now hit your Voiceflow bot.