I am having an interesting problem with our LG GD710 phones. Yes, all of them. I have 100 of them and have tried this on a dozen or so, all with the same results.
Dialing *777# for a balance inquiry returns a message of "Invalid Subscriber".
After some tinkering, I found that putting the phone in 2G Only mode through the service menu is the only thing that will make it work - it returns the correct balance info and expiration date. When the phone is in 3G Only or Dual Mode (providing the phone has had enough time to provision after bootup to establish a 3G connection), it fails with this error.
We have 200 or so LG CU720's, all with the same kind of SIMs in them, and they work just fine in either mode. Even the SAME SIM that works in a CU720, doesn't work in a GD710 (unless its's forced to 2G Only).
Tried to contact provider (Red Pocket) - they of course said "We don't support that phone. Contact LG."
LG of course said "We have no idea. Contact the provider."
And AT&T of course said "You are with Red Pocket. Why are you contacting us?"