### Answer --- Being charged on both the Retell side and the Twilio side appears to be normal behavior when using custom telephony via SIP trunking. [(1) ]() For warm transfers specifically, the behavior differs from cold transfers: - Warm transfer uses SIP DIAL, which initiates a new outbound leg to the transfer destination [(2) ]() - Cold transfer uses SIP REFER, which hands off the call differently Since warm transfer creates an additional call leg (the AI dials out to the transfer destination before connecting the caller) , this likely explains the separate Trunking Terminating and Trunking Originating charges from Twilio. Unfortunately, I don't have specific documentation on how to reduce these dual charges when using custom telephony with warm transfers. This is primarily a Twilio billing structure question. You might consider: 1. Contacting Twilio support about their billing for SIP trunk warm transfer scenarios 2. Using Retell-purchased Twilio numbers instead of custom SIP trunking, which may have different billing behavior [(9) ]()

​ To learn more, see the sources I considered: [[1] Discord message from @jake8heller on 2025-10-22](https://discord.com/channels/1186424376300679209/1186462578310910112/1430634203057360979) [[2] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) ​

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