Formatting phone numbers

Hi dear gurus and contributors
I've got a set of numbers like 0302568974025. All about more than 1000. I have to convert them into (xxx) xxx - xx - xx (Ext: xxx). But I cannot figure out how to do that. For eaxmple

In A1: 0502567899256
The first 3 numbers are mobile operator code, and always start from 0. So first number is always 0. Then 5 numbers are phone number. Last 3 numbers is external number of a person. The problem is that the external number can be 3 or 4 digits. So some phone numbers are 13, some are 14 digits long.

0502567899256 >>>>>>>> (050) 256 - 78 - 99 (ext: 256)
07745578941256>>>>>>>>(077) 455 - 78 - 94 (ext: 1256)

Hope I you understood me. I don't want to use any formula or VBA code. I need just convert them. All values are stored as numbers.