LG Ally on TnT1200 random 'insufficient funds' call disconnection

[Vchat20]

Pretty much as the title states. TL;DR: My mother has upgraded to an LG Ally that has been put on PP and she is on the TnT1200 plan with autorenew set up (and isn't due until the 14th). Not terribly often but enough to be annoying is it will disconnect her in the middle of a call and say something about insufficient funds. The actual minute balance in the plan isn't anywhere NEAR up. The only thing I can think is the dollar balance is at 0.00 but that should not affect it at all.

Any ideas? I have a Motorols Droid on the same plan and have never had the issue (albeit I don't make calls nearly as much) so it baffles me quite a bit.

[B Bell]

roaming..your balance is 0 so when the call is picked up by a roaming tower it is then droped

[Vchat20]

Thanks. I actually went in and read up the roaming stuff on PP's site (never did it originally as I half expect many prepaid providers to just not do roaming at all) and surprise, surprise: Roaming is allowed but pulls from your account balance and not the TnT balance (which would be REALLY nice. Stuff like roaming charges and international calling/texts pull from the TnT balance but at an increased rate like 3 minute deduction for every 1 minute in-call or something akin to that). I've set the phone to Home only and will be keeping an eye on it. I expect though that was the issue since the times it has had issues normally is inside the house in an area that doesn't get a decent signal on VzW.

[B Bell]

try and set the phone to the home only network and it shouldn't pick up any roaming towers