First Direct don't know area codes
Web developers never seem to understand email address RFC's , and I've had my own problems with standard domains such as ".info" (if it's not a .com,.net or .org so it is not an email address, apparently) - now it's how phone numbers are allocated.
My latest experience with First Direct, (a bank owned by HSBC so the web developers should have the money and time to investigate these things) - it would not let me enter the area code for London the capital city of the UK. I would assume it is well known for its dialling code of "020" (it was changed 15 years ago, after all) ,and refuses to accept it, stating that my "work number area code should be between 4 and 6 digits".
It's not like the information is hard to find, Wikipedia has a large entry for "Telephone numbers in the United Kingdom" and even has a section on "Two digit area codes" (the article ignores the 0 prefix for its purposes), which includes other cities such as Southampton & Portsmouth, Coventry and Cardiff, so I assume people in these cities also can't put their telephone number area code in properly. These are hardly tiny, unknown areas, but major metropolitan cities.
Well done First Direct!