When cell systems were developed industry had to develop a priority system to make sure that first responders and national security (politicians and their significant others) had priority access and could get calls through during peak usage times.
TDMA and original CDMA technologies did have this prioritization technology.
GSM, Iden and current CDMA do use a prioritization technology.
So why does AT&T suck?
Simple, essentially when a different phone company leases use of one phone companies towers the users are both in the same technical priority group.
But AT&T has a problem, 3G networks and the iPhone.
To keep primary customers happy AT&T has resorted to prioritizing their customers over their competitions customers in violation of FCC regulations.
Because the data connectivity of AT&T customers is clogging the system. If AT&T gives the same priority to T-Mobile that it gives to AT&T the AT&T customers would get a reduced level of service so AT&T prioritizes their customers communications over the competition which means if you are using a non-AT&T system, or even a pay-as-you-go AT&T system, you are hosed when you live in an area where AT&T controls access.
These guys were busted up once for unfair business practices and here we are again, forty years later and the morons are doing the same thing over again with a new technology.
I would watch the news, sooner or later the evidence is going to surface that AT&T is taking advantage of the Priority Access Dialing and the Cellular Priority Access System to improve their profits at consumer expense.
When that hits I would short sell AT&T (long term) and buy stock in competitors since AT&T is headed for another break-up and this second time around time people are going to be so pissed that the name AT&T will probably be outlawed.
You can't fix stupid and morons never learn.