Didn't find the answer by searching through this sub. U.S. Phone number incorrect to verify on Prolific?

Hi All! based on some posts on this sub, I joined prolific. When I try to enter my phone number it comes up as invalid. I see in the note it is asking for the formatting typical of UK phone numbers. I was under the impression people in US can also use this site? How did those in the US verify your phone numbers? Thanks!