I also searched for it some time ago, and there is not reliable way to do it
PhoneId.GetLine1Number can work in some cases, but it can be empty, or even worse, hold a different number.
The only way I know of is two-step validation, writing your number in the app, sending it to the server, and then it sends an SMS to this number with a validation code which can be automatically captured by your app or read by the user and then entered into the app.