How can we improve WHMCS?

We use a custom field to specify the mobile phone of the customer.

This field is used to sent notifications (invoice, promo, etc...)
Now, with the new phone number system, do you think is it possible to integrate the same option (to add a phone) in custom field ?
Cause the customer who specify its phone doesn't understand why in the default phone number he doesn't need to add prefixe like 0032.... or +32.... for the country and in custom field he needs.
Thanks for consideration :)