### Answer --- Based on the available documentation, the Update Phone Number API does expose the

inbound_agent_id

field, which you can use to bind an agent for inbound calls. [(1) ]() You can update your phone number via the API with: -

inbound_agent_id

— Unique id of agent to bind to the number. The number will automatically use the agent when receiving inbound calls. If null, this number would not accept inbound calls. -

inbound_agent_version

— Version of the inbound agent to bind (optional, defaults to latest) . For custom telephony/SIP numbers, ensure: 1. The number is properly imported via the Import Phone Number endpoint. [(2) ]() 2. The

inbound_agent_id

is set to your desired agent. [(1) ]() However, I don't have documentation on specific SIP-level configuration fields beyond agent binding. If inbound INVITEs are returning "Busy" despite having an agent assigned, this may require support intervention. Recommended next steps: - Visit [Contact Sales]() for direct support assistance with your custom telephony configuration. Would you like me to try providing a general answer based on my knowledge?

​ To learn more, see the sources I considered: [[1] Update Phone Number](https://docs.retellai.com/api-references/update-phone-number) [[2] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.