You can call like this:
if let url = URL(string: "tel://\(number)") {
UIApplication.shared.openURL(url)
}
For Swift 3+, you can use like
guard let number = URL(string: "tel://" + number) else { return }
UIApplication.shared.open(number)
OR
UIApplication.shared.open(number, options: [:], completionHandler: nil)
Make sure you’ve scrubbed your phone number string to remove any instances of (, ), -, or space.