I had the situation where Microsoft Teams is configured for Direct Routing and calls are dropped unexpectedly. We are using an Audiocodes SBC in Microsoft Azure and a SIP Trunk from Deutsche Telekom for the PSTN connection.
Sometimes an inbound call is signaled by Microsoft Teams for a second and then, it is dropped. In the Microsoft Teams activity feed a missed call is visible. Also, a missed call was shown in the activity feed even there was no ringing.
At first, I thought I could bring it down to cell phone calls only. But after asking my colleagues some users reported the same behaviour for calls coming from PBX systems.