Great question — answering as the UphillTech team:
We have some similar cases here. In these cases, we use Twilio conference instead of transfer call. We include the A and B end of the call in a conference. To change participants, with the conference, we have the freedom to delete or add participants https://www.twilio.com/docs/voice/conference
Full answer by Leticia Duarte: https://stackoverflow.stackexchange.com/a/78496683 (CC BY-SA 4.0).
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