Cingular in NJ/NYC: Can you roam on T-Mobile?

I just changed to a 64K SIM card and now I can't roam on T-Mo anymore. I can only use the blue network. I go to my network selection menu, select T-Mobile and it doesn't let me register. However, I think those with Cingular 32K SIM cards can still use T-Mo towers. If you have a 32K SIM card and you are in North NJ or NYC, can you use the T-Mo network?

I really don't miss T-Mobile coverage that much (who would in this area? ) but it is just nice to know it is there as a backup just in case one day the Cingular blue network misbehaves which has happened a couple of times here.

Note: You can find out which towers you are using by dialing #121. If you get a recorded message saying that "your call cannot be completed as dialed", then you are using T-Mo towers. However, if you get a greeting from "Voiceinfo", then you are using blue towers.

 

In GSM there are small regions called LACs and that's how roaming is controlled. If your SIM is allowed to roam on another carrier, then all the LACs in the area in question must allow any SIM card from the other carrier for successful network registration to occur. Apparently, T-Mobile has some way of blocking certain Cingular SIM cards on the T-Mobile LACs in this area. SIM cards don't have a PRL type of list to control which networks can or cannot be used. It is all done in the network side, so there are no updates to take from the network. The only thing GSM SIM cards have is a network priority list which mandates which carriers the phone should look for first when scanning for service.

Now, the answer to why Cingular wants to block roaming on T-Mo is very clear: Roaming costs money and when Cingular and T-Mobile ended the sharing agreement in January this year, Cingular agreed to pay T-Mobile for every minute Cingular users spent on the T-Mobile network.