An encapsulation of the act of placing a call on hold or removing a call from hold.
- iOS 10.0+
When a caller places a call on hold, callers are unable to communicate with one another until the holding caller removes the call from hold. Placing a call on hold doesn’t end the call.
When the user or the system places a call on hold, the provider sends
provider(_: to its delegate. The provider’s delegate calls the
fulfill() method to indicate that the action was successfully performed.