Describe the feature

As of release of realtime-gpt model, OpenAI Introduced the SIP connections already.
Would be awesome to:

1. Set the SIP Connection with the OpenAI Agents SDK
2. Connect the SIP conversation with the OpenAI Agents SDK realtime conversation engine

Twilio already released a guide on how to use this with their Elastic SIPs:

https://www.twilio.com/en-us/blog/developers/tutorials/product/openai-realtime-api-elastic-sip-trunking