如何在使用Callkit时播放声音，如铃声？

import UIKit
import CallKit
import AVFoundation

class ViewController: UIViewController {
    var audioPlayer: AVAudioPlayer?

    // ...

    func startCall() {
        let callUpdate = CXCallUpdate()
        // 设置来电铃声音频文件
        callUpdate.remoteRingbackURL = URL(fileURLWithPath: "path/to/ringtone.mp3")

        let callHandle = CXHandle(type: .generic, value: "Incoming Call")

        let startCallAction = CXStartCallAction(call: UUID(), handle: callHandle)
        startCallAction.isVideo = false

        let transaction = CXTransaction(action: startCallAction)

        let callController = CXCallController()
        callController.request(transaction, completion: { error in
            if let error = error {
                print("Error starting call: \(error.localizedDescription)")
            } else {
                // 开始播放铃声
                self.startRingtone()
            }
        })
    }

    func startRingtone() {
        guard let ringtonePath = Bundle.main.path(forResource: "ringtone", ofType: "mp3") else {
            print("Ringtone file not found")
            return
        }

        let ringtoneURL = URL(fileURLWithPath: ringtonePath)

        do {
            audioPlayer = try AVAudioPlayer(contentsOf: ringtoneURL)
            audioPlayer?.numberOfLoops = -1 // 无限循环播放
            audioPlayer?.play()
        } catch {
            print("Failed to play ringtone: \(error.localizedDescription)")
        }
    }

    // ...
}