Simple function to prompt a phone call on iOS in Swift

makePhoneCall(phoneNumber: String) .swift

func makePhoneCall(phoneNumber: String) {
	if let phoneURL = NSURL(string: ("tel://" + phoneNumber!)) {
		let alert = UIAlertController(title: ("Call " + phoneNumber! + "?"), message: nil, preferredStyle: .Alert)
  		alert.addAction(UIAlertAction(title: "Call", style: .Default, handler: { (action) in
  			UIApplication.sharedApplication().openURL(phoneURL)
  		}))
  
  		alert.addAction(UIAlertAction(title: "Cancel", style: .Cancel, handler: nil))
                
  		self.presentViewController(alert, animated: true, completion: nil)
	}
}

Small update for Swift 5.0 :)
`
func makePhoneCall(phoneNumber: String) {

    if let phoneURL = NSURL(string: ("tel://" + phoneNumber)) {

        let alert = UIAlertController(title: ("Call " + phoneNumber + "?"), message: nil, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "Call", style: .default, handler: { (action) in
            UIApplication.shared.open(phoneURL as URL, options: [:], completionHandler: nil)
        }))

        alert.addAction(UIAlertAction(title: "Cancel", style: .cancel, handler: nil))
        self.present(alert, animated: true, completion: nil)
    }
}

`

Working like charm.
just for someone who is a beginner and not able to call.
pass the function in button, make an action connection from storyboard to view controller.

@IBAction func btnCall(_ sender: UIButton) {
makePhoneCall(phoneNumber: "1234567890")
}

when entering this line of code > > makePhoneCall(phoneNumber: "1234567890"), 'expected parameter type' error message occurs, new to coding and unsure what to do, Regards

@lplazas just call the UIApplication.shared.open(_:) it will run the native phone call alert. That could work if your just want an alert with only one number.