what is the model on the phone?
It is either a tdma phone or a gait phone. Cingular is getting rid of that system... and even though the phone is still working, a tdma phone (old system) will not work on a
gsm (new system) plan.
Now, another problem may have been that Best Buy can only do new activations (that's what your phone would have been), but cannot do upgrades (ie your dad's phone - he is already with cingular). What you would probably have to do is go to a cingular store to upgrade your dad's phone, then go to best buy & add your line (you will get the razr for about $100 cheaper @ best buy vs. a cingular store).
I guess I need to know what the phone model is... because if it is a nokia 6340 or a sony ericsson t62...there may be a way around it...