The GSM rollout in the Lakes Region has been painfully slow, but has made substantial progress in the past 3-4 years. Originally, the only GSM service in the area originated from over near Laconia, so folks on the eastern side of the Lake using services such as AT&T/Cingular were subject to crappy (most non-existent) signals. Verizon was much stronger in the Tuftonboro area (only reason I switched from it was when my company moved into a new building that acted as a Verizon "anti-antenna" -- total dead zone).
Heading up from Concord, GSM users still can count on losing signal once you pass the Epsom circle, and not really gain a meaningful signal again until South Wolfeboro. Head north out of Wolfeboro on 109 and you'll lose signal before the cemetery. You can get sporadic GSM signal on 19-Mile-Bay, but lose it completely as you hit the hill heading north. Abenaki Tower - great signal
...then nothing till Moultonboro. I've had good GSM signal all over the Lake in a boat.
It's hard to say what impacted the Verizon signal...their coverage, although it's a proprietary network, is darn good. The truly baffling thing is the lack of GSM service, given the number of carriers who use it.
And, no, I don't make a habit of talking while driving on 109, 28, 25, 16, 109A, etc., but have paid attention to signal strength & availability up there over the years.