Hi there,
T-mobile use the 1900MHz repeater uniformly across the country, whereas Cingular use either the 800MHz or 1900MHz depending on the location. If you friends phone is a tri-band form Europe then it is likely that it only works on the 1900MHz band. If this is the case, then I am afraid that there is nothing you can do apart from buy a new phone. If however your friends phone is a US phone then is should definitely work.
It may be that you are in a poor signal area - can others on Cingular get reception in your area. If not, then you might need a cell booster to boost your signal.
If others are getting Cingular signal and you are seeing nothing, then there might be a problem with your phone or SIM card. I'd suggest taking your phone to a Cingular store as they will be able to advise you on what is going wrong.
Hope this helps,
Sam