Dear there,
I read your requirement and found that we can help you.
If you are developer, I suggest you can try to split telephone number in 2 fields in front end and put current telephone filed in hidden. So, you can change the entry via javascript for current telephone number (combination of both prefix and telephone). So, user can get 2 fields in frontend and magento will work as they are working and stored number as single field in database.
Please let me know if you have any query.
Kind Regards,
Taha
MoonRose Infotech