I have the same issue and reproduced it with the Apple demo:
https://developer.apple.com/documentation/callkit/making_and_receiving_voip_calls_with_callkit
and I reported an feedback to Apple, still no response yet.
Steps:
Device: iPhone12 pro max - iOS: 15.1.1
make a VoIP call with the demo, the VoIP call is active
make a native outgoing call, then the VoIP call becomes an on hold state.
End the native call
by this side: the system "didActivate" callback comes, then the VoIP call becomes active again. (expect behavior)
by the other side, the system "didDeactivate" callback comes, then nothing changes to the current VoIP call. still on hold state. (abnormal, bug behavior)