That shows you do not have a Mobile Number for me. Quite correct as I don't have a mobile phone...
Click Edit, page is asking for a "Telephone Number". I provide one only to be told it's not a valid Mobile number. I know it isn't - I don't have one... (but it is a perfectly valid land-line number). You should also note that the page cannot be submitted unless both entries are valid (non-blank).
Bit of a problem there, no? You're asking for something I can't provide and are refusing to accept the only viable alternative I can provide!
To compound the problem you're currently using the email address I originally provided to nPower. That email address was created for, and only ever issued to, nPower (it's a unique address). You are, obviously, not nPower and therefore you too should have a unique email address created for your use alone. Unfortunately I can't provide one because of the problem(s) with the 'phone number described above.
Suggested solution - have both land-line and mobile number entries (possibly with a "Preferred" radio button by each, if both are provided).