Default SIM - Data works, Calls and Texts do not

Hi,
I followed the sticky guide from the general section - I'm able flash roms, and cwm, root, etc.
However
Although I have adequate signal - i'm able to use 3g data - I can't make or recieve calls or texts.
Everything seems to be working right up until dialling a number - when the phone immediately hangs up.
I've spent quite a few hours over the last couple of days looking round XDA for similar problems - and the issue (phone immediately hangs up but data is working) seems relatively common - but no joy yet - except for trying a new rom, toggling data on and off, and a new motherboard.
I've tried several roms, including an out-of-region stock, and formatted everything except the sdcard, and nothing is solving the issue.

I'm guessing I've mashed up something to do with the radio- as I think it's not starting - but I don't understand how flashing a new rom doesn't replace the corrupted files and resolve the issue.

is there a way of formatting absolutely everything and then reinstalling everything from scratch?

Will a phone repair shop on the high street be able to flash something I can't flash over usb?

Further details: opening call forwarding settings gives a "Call settings error - network or sim error" message
likewise "Caller ID" etc.
The IMEI is displayed correctly, but the phone does not show my phone number.