Hello, I'm a NOC VOIP specialist with 10 years experience working for a US voip company, I have a lot of experience troubleshooting, installing and customizing all sort of VoIP equipments and PBX systems based on SIP; Please note I'm not an Android developer, but I read your requirements and saw your video, and I'm offering to setup a live trace between your Android app and twilio so we can get a full PCAP from your Android softphone, it's important to analyse the packets between your Android and the termination partner, from your problem description it sounds like the RTP stream cuts of for some reason, it could be ISP level, app level(sip settings) or network level(Nat/firewall). Feel free to contact me if you're interested on my approach to solve this problem, since I'm not an Android developer and will not be able to make changes on your swift code, I'm open to run these tasks for you for a lower price, and we can discuss more about it on chat.
Thank you!