The Country Code of the mobile phone number is based on the users current geolocation (the country where you are currently) and cannot be edited by the user for security reasons.
How can I complete my KYC if I have a phone mobile phone number from a different country than where I can currently?
1. Register (select nationality based on your ID Docs) as usual
2a. Get a local phone number to use with verifying your account and follow steps as usual
2b. Send an email to email@example.com giving them your Client ID and kindly asking them to manually add your mobile phone number to your account
3. Complete KYC verification steps and when you get to phone number verification, then it has already been added and follow steps as usual