Uni Compare: Degree Courses UK

I just downloaded this app cause I’m really interested in researching schools as an American, but the app won’t even let me create an account because it says my mobile number is incorrect and when I click on the help icon to tell me what’s wrong with it, it just tells me it’s not valid. How am i supposed to fix it if your app doesn’t tell me how exactly it wants it formatted. I tried so many ways that you can format a phone number and none worked. It’s seriously annoying and frustrating like wth.