if you just barely have enough signal to place a call, IMO that could be the problem....
Before I upgraded to an iPhone and changed postions, I had a droid, where I worked, it fluctuated between 1-2 bars, sometimes no signal. It was nothing for me to unplug fully charged phone, drive 5mins to work, 4-5hrs of work and 5-6 text messages later phone was shutting down due to dead battery. On weekends with no work, 20mins of talking, 10 texts or so, 20-30mins of playing games (*not angry birds*) 10-12 hours of stand by, battery ranged from 40-48%
* now onto angry birds* I don't know why, but I could run the droid down from fully charged to 20% by playing angry birds for about 40mins non stop....