Sorry if i am duplicating this!!
I got the BB from T-mo about 5 days back and called tmo to register the PIN on their network but they transferred me to RIM and RIM told me that they has known issue on this registration for 8700g devices. It has been almost a week by now and so solution yet.

Wonder, if anyone has success using 8700g so far on t-mo.. I have 8700c and it iw working great but need to see g working on too..

painfull!!