I have seen the same problem, however it is not with everyone GSM phone, and it appears to be only with Cingular. I am not sure why this is. I have t-mobile and a RAZR and it does not cause the same problem as friend who has Cingular and RAZR. We swapped phones and changed out SIM cards and the problem stayed with Cingular.
Interesting part of the interference is it only seems to happen right before the phone rings, it is like it boosts its RF energy to recieve the call. So everytime the phone is about to ring you hear the interference and I say to my friend your phone is about to ring.
It's quite simple, and it's the same kind of issue as Sprint PCS vs Verizon, both of whom also use the same technology but have radically different reputations for quality.
Both T-Mobile and Cingular use GSM. However, GSM can be implemented in a variety of ways. Cingular (both Orange and Blue - AT&T) started off with AMPS networks. These were then upgraded to D-AMPS, solely to increase capacity. D-AMPS, frankly, had no future, and Cingular upgraded to GSM to get it on the path that eventually leads to UMTS (3G GSM) and enlightenment.
There was an issue however. Cingular had managed the growth of its network by simply choosing a more efficient technology, D-AMPS, to carry calls. D-AMPS gives 3x as much capacity as AMPS. So just by upgrading its towers, you get 3x as many calls per tower. GSM doesn't work like that. GSM's advantage over analog from a capacity point of view is that it allows you to use smaller and smaller cells, covering an area with more towers, without interference. (It also manages interference better, so you can use more of the spectrum.) This makes a certain amount of sense, but if you're upgrading from D-AMPS to GSM, you immediately go back to having nearly 1/3 as much capacity as you did to start with.
Enter Half Rate, the cheap band-aid for overloaded GSM networks. Half rate uses a more efficient codec (there are two, HR, and AMR-HR) but in exchange for not terribly good call quality when reception is good, and lousy quality (barely usable) when reception is bad. This, however, improves capacity 100% as two calls can be squeezed into the same space as an ordinary GSM call, so you're now down to only 2/3 as much capacity as your old D-AMPS network, rather than 1/3.
Cingular started running out of capacity, so they switched much of their network to AMR-HR. And that's why Cingular isn't that great at the moment.
T-Mobile, on the other hand, started off as a pure GSM network. It's been fairly sane in managing its growth, so it hasn't needed the HR "bandaid" and call quality is pretty good, considering.
Verizon and Sprint PCS also have the same issues, albeit for different reasons. Verizon has managed its growth well (and only went from AMPS to C-AMPS (IS-95), so hasn't faced a sudden drop in capacity). Sprint PCS, on the other hand, seems to have a phobia about putting up new towers, and believed the hype at the end of the 1990s about CDMA having "infinite" capacity and other such nonsense. So call drops, "ballooning", and poor quality, at peak periods seems to be fairly common for Sprint PCS users.
The current two technologies used throughout the US, GSM and IS-95, are generally very good in quality terms when properly implemented, and absolutely diabolical when the operator is "cheap".
As an aside, you'll note that this message will get a bunch of replies along the lines of "Well, I use Cingular and it's awesome!", "T-Mobile is teh suck!", "Sprint PCS never drops calls on me!", "Verizon's awful". This is because all these networks have areas where they're well implemented and others where they fall below common expectations. On average though...
