Try this for E710, haven't tried it on E710, but works on many other samsungs
. Turn off your phone, put in a SIM of a provider that it is not locked to, and turn it back on.
2. At the wrong card message, type *#9998*3323#. A message will pop up telling you to press exit, so hit the right softkey. Choose number 7 (or 8 or 9 work, I think). Your phone will reboot and your SIM will work.
3. Type *0141# and then hit call. This will give you a message saying your phone is personalized.
4. Power down your phone and insert a SIM card other than the one that is in it. Power back up your phone.
5. At the SIM code prompt type 00000000, that is 8 zeroes