I made an internal call via fritz.box and I called myself via the telephone by **600. I heared the ringtone but when I accepted the call, neither of us did hear anything. I'm using Kubuntu 18.04. I guess, these are the important lines in the log:

[1577674451.545|19511|sipcall.cpp       :906  ] [call:3558973192781136] startAllMedia()
[1577674451.545|19511|sip_utils.cpp     :203  ] Registered thread 0x7ff0bdff6dd0 (0x4C36)
[1577674451.546|19511|sdp.cpp           :599  ] Could not find rtpmap attribute
[1577674451.546|19511|sdp.cpp           :599  ] Could not find rtpmap attribute
[1577674451.546|19511|sdp.cpp           :616  ] Could not find codec G726-32
[1577674451.546|19511|sdp.cpp           :616  ] Could not find codec G726-32
[1577674451.546|19511|sdp.cpp           :616  ] Could not find codec G726-40
[1577674451.546|19511|sdp.cpp           :616  ] Could not find codec G726-24
[1577674451.546|19511|sdp.cpp           :616  ] Could not find codec iLBC
[1577674451.546|19511|sdp.cpp           :616  ] Could not find codec telephone-event
[1577674451.546|19511|sipcall.cpp       :956  ] [call:3558973192781136] [SDP:slot#0] Missing remote codec
[1577674451.546|19511|video_rtp_session.cpp:91   ] No socket pair, keyframe request callback not possible

Full log: https://pastebin.com/RtTvcmBY

Kind regards