Great question — answering as the UphillTech team:
It looks like you may have a participant bound to a previous covnersation. A participant is uniquely tied to a specific conversation using a unique pairing of their mobile phone number and a Twilio proxy conversation number. If the participant is already in an existing conversation, then you need to use another Twilio proxy conversation number for this new conversation you want to add them too. This will also create a unique thread on their phone (which only makes sense, since they are separate conversations). I suggest listing the existing conversations to see which conversation they are already a participant of (using that Twilio Proxy number), and remove them. I find using the Twilio CLI...
Full answer by Alan: https://stackoverflow.stackexchange.com/a/67040395 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
