iPhone，拨打另一个电话号码以响应第一个电话号码未接听

如何在iPhone上拨打另一个电话号码以响应第一个电话号码未接听？

swift
import CallKit
class CallManager: NSObject, CXCallObserverDelegate {
    let callObserver = CXCallObserver()
    
    func makeCall(phoneNumber: String) {
        let provider = CXProvider(configuration: CXProviderConfiguration(localizedName: "My App"))
        provider.setDelegate(self, queue: nil)
        
        let update = CXCallUpdate()
        update.remoteHandle = CXHandle(type: .phoneNumber, value: phoneNumber)
        
        provider.reportOutgoingCall(with: UUID(), startedConnectingAt: nil)
        provider.reportOutgoingCall(with: UUID(), connectedAt: nil)
    }
    
    func callObserver(_ callObserver: CXCallObserver, callChanged call: CXCall) {
        if call.hasEnded {
            // 第一个电话已结束，可以拨打第二个电话
        }
    }
}