LaF0rge@perillamint @tnt is working for quite some time on something related; its surprisingly hard and there's very little FOSS in this area. With spandsp and his hacks/fixes we can get V.22/V.22bis(1200/2400 bps) working, but with unreliability. His primary use case is driving an AMR reserviert card (MC97) via an icE40 FPGA. So not sip but actual telephony audio interface. Once that works, it should be possible to attach SIP/RTP.
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