I don't know if i am barking up the wrong tree?
but i have noticed that the head unit uses 900mhz Gsm and the modern ones use 2100mhz.
Does this mean up to and including and backward compatible, or are they two seperate systems?? head getting cabbaged now!!! 
I think the first wave of GSM phones were 900MHz (Vodafone/Cellnet) and the second wave were 1800MHz (121/Orange).
3G (third generation) mobile phones I think use 2100MHz and different technology to the GSM phones.
The phone in the Omega can only work with 900MHz. If you get an antenna that works with lots of frequencies thats fine just as long as 900MHz is one of them.
So Vodafone and Cellnet (became O2) or any company piggy backing off their networks should be fine so long as it isn't a 3G sim card.
Note - I think 1G were analogue, 2G means GSM/digital and 3G means newer faster/digital. The telematics unit seems to be limited to 900MHz GSM.