I've run Speakerbox(https://docs-assets.developer.apple.com/published/8e99045a90e2/MakingAndReceivingVoIPCallsWithCallKit.zip) to check the behavior of CallKit.
When there was one active call and one on hold and another call was received, the behavior was defferent depending on whether I operated it from a headset or not.
No headset or operated from CallKit screen
When "End & Accept" tapped: the active call is hung up, and the received call is answered.
When "End Held & Accept" tapped: the held call is hung up, the active call is held, and the received call is answered.
Operated from headset
Answer opeation cannot be performed for unknown reasons
Disconnect operation caused that all calls other than the one on hold are hung up and the held call is unheld.
Hold operation caused that the active call is held and the received call is answered. (Strangely, there are two held calls.)
And when I toggle calls at this time, one of the two on hold is hung up for unknown reasons.
I tried changing the settings of CXProviderConfiguration and CXCallUpdate, and changing the options in the Audio Session category, but it did not improve.
I checked CXActions occurring, and it was as follows.
(Call A = held call, Call B = active call, Call C = received call)
Disconnect (or Answer?) operation
(lacking CXEndCallAction for Call A)
CXEndCallAction for Call B
CXAnswerCallAction for Call C
CXEndCallAction for Call C <- weird
CXSetHeldCallAction for Call A onHold=false
Hold operation
(lacking lacking CXEndCallAction for Call A)
CXSetHeldCallAction for Call B
CXAnswerCallAction for Call C
Toggle calls
CXSetHeldCallAction for Call C onHold=true
CXSetHeldCallAction for Call A onHold=false
CXSetHeldCallAction for Call B onHold=false <- weird
CXEndCallAction for Call B
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