CallKit：在 GSM 通话断开后恢复保持的 VoIP 通话

Question

I'm experiencing an issue where I'm unable to resume an on hold VoIP call after a GSM call is disconnected by the calling person.

Scenario one that works fine :

1. I answer an incoming VoIP call
2. I get a GSM call, press 'hold and accept', which results in provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) being called, allowing me to put the VoIP call on hold
3. I hang up the GSM call which results in provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) being called, allowing me to un-hold the VoIP call

Scenario two :

1. I answer an incoming VoIP call
2. I get a GSM call, press 'hold and accept', which results in provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) being called, allowing me to put the VoIP call on hold
3. The caller hangs up the GSM call, but provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) is not called

In scenario two only provider(_ provider: CXProvider, didDeactivate audioSession: AVAudioSession) is called after the caller hangs up the GSM call.

How can we resume a VoIP call after a GSM is disconnected by the caller if provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) is not called?

public func provider(_ provider: CXProvider, perform action: CXSetHeldCallAction)
{
    print("Callkit: provider(_ provider: CXProvider, perform action: CXSetHeldCallAction): \(action.callUUID)")
        
    guard var call = callManager.call(withUUID: action.callUUID) else {
        action.fail()
        return
    }
        
    call.isOnHold = action.isOnHold
    call.isMuted = call.isOnHold
        
    audioRouter.audioDevice.isEnabled = !call.isOnHold
        
    action.fulfill()
}

Answer 1

After A GSM call is ended, applicationDidBecomeActive will be called when returning to the app. At this point we can detect if a call is on hold and resume it. Other solutions welcome.

Answer 2

I ran into this today. After some Googling, I was unable to find a solution. So strange, I thought, that nobody has solved this. I was able to find a solution inspired by @Simon, but I think a bit cleaner.

In your CallViewController, AppDelegate, or similar, set yourself as the CXCallObserver's delegate.

CXCallController.shared.callObserver.setDelegate(self, queue: .main)

Then later, you want to detect if there is exactly one active call, it's "yours" (whatever that means for you. For me it's stored in self.call.id ) and that it's on hold. If so, you can request an update to "unhold" it, and your code elsewhere that handles func provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) will get invoked, and hopefully your app handles it correctly.

  //MARK: - CXCallObserverDelegate
  
  func callObserver(_ callObserver: CXCallObserver, callChanged call: CXCall) {
    guard let activeCall = callObserver.calls.first,
          1 == callObserver.calls.count,
          activeCall.uuid == self.call.id,
          true == activeCall.isOnHold else {
      return
    }
    let transaction = CXTransaction(action: CXSetHeldCallAction(call: activeCall.uuid, onHold: false))
    CXCallController.shared.request(transaction) { error in
      if let error = error {
        //handle it?
      }
    }
  }

When you get an incoming call, from say, WhatsApp, and your app is put on hold, CXCallObserver.shared.calls will return two calls. One will be "yours" on hold, and the other will be "the WhatsApp call" not on hold. Once the WhatsApp call is terminated on the other end, CXCallObserver.shared.calls will only return one call, yours, but it will still be on hold. This seems like a fairly egregious bug in CallKit. But what do I know?