Whatever API they are using the get the phone number without the user entering it on mobile is probably easy to spoof. It may not even work on all platforms and

Whatever API they are using the get the phone number without the user entering it on mobile is probably easy to spoof. It may not even work on all platforms and the user may have to type a phone number in that case. Would you trust a phone number handed to you by a PhoneGap JavaScript method, for example? It would be pretty easy to tweak to return whatever is needed.