Phone call registry returning an optional phone call

PhoneCallRegistryOptionalPhoneCall.swift

protocol PhoneCallRegistry {
    func callWithIdentifier(identifier: Int) -> PhoneCall?
}

func hangUpCallWithIdentifier(identifier: Int) {
    if let call = phoneCallRegistry.callWithIdentifier(identifier) {
        call.hangUp()
    } else {
        print("Could not hang up call")
    }
}