Invalid mobile phone number

Not sure if this is right place to post this. A website i'm using has sent me a link to a form I need to fill in, that uses adobe sign. The first step is the 'government id verification' step, which asks me to enter my mobile phone number but when I do it just says 'enter a valid phone number'. This is a UK number and I've tried with and without the leading zero, but it still won't accept my number.

Does anyone know why this might be happening? I thought these things just use a regex or something similar to validate it looks correct, so either their regex is wrong or it's doing something else.

Thanks