Voice calls over Virtual Um

Hi, I set up a network with Virtual Um and multiple OsmocomBB phones. They register, they can send and receive SMS and I wanted to try voice calls. I looked around and found a repository [1] with an MNCC example application which sends voice samples. Calls are established and can be picked up, but on the other end, but I never seem to receive a GSM_TCHF_FRAME-message on the MNCC socket. I then checked out the fixeria/mncc branch [2] which includes GAPK integration which can use ALSA devices for audio, because that's what I need anyway at some point for my project I'm working on. Again, calls can be established, but GAPK is never initialized and again, I never see voice data appearing as an MNCC message. The GAPK audio backend is supposed to be initialized [3] when a CHANNEL MODE MODIFY message is received from the network. I never see the network request a channel mode modify though. I don't know enough about GSM to find out if that message is always supposed to be sent when a call is established or not. Did anybody ever transport audio with two OsmocomBB instances over the Virtual Um interface? Thanks, - Andy [1]: https://github.com/GerardPinto/call-control-mncc-sap [2]: https://git.osmocom.org/osmocom-bb/log/?h=fixeria/mncc [3]: https://git.osmocom.org/osmocom-bb/tree/src/host/layer23/src/mobile/gsm48_r…