What you may want to consider doing is going to the store that you have been purchasing/exchanging these devices through and asking the rep who has been helping you to submit a trouble ticket the the network ops people concerning your house. Nobody really thinks those reports do anything, but a co-worker of mine once submitted enough tickets for the same customer about calls dropping while in his house that they sent out a team to reposition a piece of tower equipment, and that solved the problem. If you're running out of your 30 days, then perhaps other reps can suggest another handset, but it sounds like you've gone through the likliest candidates already, and maybe the above suggestion can help you out. Best of luck.

Have you had your sim card check out?