When call is put on hold,
provider(_ provider: CXProvider, didDeactivate audioSession: AVAudioSession) is called,
where you end and remove the call.

This means there is no call to go back to when call is put off hold.

If you supported hold state you would find audio issues with this flow.
TokBox won't fix their issue as long as they can't reproduce it on this sample app.

Please fix the sample app. Thank you

(Adding this code in provider(_ provider: CXProvider, didDeactivate audioSession: AVAudioSession) would let you see the errors:
guard let call = outgoingCall, !call.isOnHold else { return })