Great question — answering as the UphillTech team:
Zoiper works fine as it uses TCP/TLS/UDP transport. Sip_ua uses websocket ws/wss transport, which is not supported by Twilio: Twilio sip call is not working from tryit.jssip.net . As alternative solution check Siprix flutter plugin https://github.com/siprix/FlutterPlugin , which can make SIP connections over TCP/TLS/UDP and uses webrtc for media.
Full answer by SipDev: https://stackoverflow.stackexchange.com/a/79054037 (CC BY-SA 4.0).
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