iCloud purchase form asks for area code for mobile number

To bypass this form error, split your mobile number into two parts so that the system sees a standard telephone format. 

Apple's payment and billing forms are built on an old system designed for fixed landline numbers, which always require a separate area code. Because the form requires you to fill out both fields to proceed, you have to "trick" the system into accepting your mobile phone details. 

In the U.S., all phone numbers, mobile or landline, have area codes. If you're seeing that and that's not the format for numbers in your region, double-check that you've got the region set properly.

For a standard 10 digit phone number (following the country code, which might be omitted, especially in the US,) a in a number like 2145551212, the first three digits (214) are the area code. The next three (555) represent the exchange--at least in the US.