The only times i had dropped calls problems were during times when they were making big changes to the network. I think the only time that caused dropped calls though was during the install of the 850mhz 3g overlay, when 1900mhz 3g network existed. The reason for this is because they ripped down the site antennas to replace them with 850mhz capable ones, so there was site downtime which led to congestion problems/network degradation from time to time. After they were mounted there may have been an issue here and there from them doing stuff etc for the install.
To be honest i didn't have any problems dropping calls on GSM when they were installing 3g. They did have to rip down the GSM antennas (one per sector at the time) to install mounting racks to install the additional antennas for 3g though. I noticed a degradation of signal at the time, but didn't have any problems, if signal was lost enough, we would have just flipped over to Centennial.
One thing I did notice at the time (not long after) was some degradation of call quality as they reduced GSM channels to clear spectrum for 3g. The most noticeable thing was that the hard handoffs went to **** from some reason, even in FR as before then they were transparent. Our market at the time only have 15mhz of PCS (one way) so we lost about a third of what was running as they cleared the uppermost 5mhz for 3g.
Now adays, calling has been great. I do make a lot of 20-60minute calls to friends, and now with the 850mhz overlay in action, I never drop down to GSM.
GSM though has been good lately too. My mom has a w518, and whenever i talk to her over the phone, the sound quality is really nice sounding, very full in range. heh i'm always kinda shocked by it to be honest lol.
The main issue with AT&T and quality issues is overloaded 3g primarily because it's deployed in spectrum that has to be shared with legacy GSM. Back when the 3g came out, AT&T only had single channel of 1900mhz lol. I think back they weren't even allowed to shutdown AMPS till Feb of that year, so i'd imagine there was a lot of work getting that old stuff out of the way, there was also TDMA they had to shutdown too.
Now they have been working on getting more channels up, I've seen 3 channels down in Indy and DC which is good. Most places should have at least 2 channels, but some areas may be spectrum starved. Hopefully more and more of the sub base is 3g capable so they can turn down more GSM to flop more spectrum to 3g.
And yes, increasing site density helps as well, and I have seen them work on that as well.
All in all, over the past 3 years i've seen them do a lot, sometimes it seems like it could have been faster and or more, but at least the progress has been steady.