If you don't receive the text message to register then please make sure that you are entering the full phone number without any other characters.
You don't need to enter the international code either, we get this from Country field on the registration page.
For example if you are in the UK then you should enter your mobile phone number as
07777123456
Numbers that wouldn't work would be
07777-123456
07777 123456
0044777123456
+447777123456
If you still have problems please open a support ticket here on the support portal.