There might be two possibilities for this problem. One is the previous owner would have reported his mobile is stolen or lost through the network provider. So the mobile must have been blocked by the network provider. A network blocked cannot be unblocked by individuals. Only the network provider can release the block after the verification of original owner information.
Second is the mobile must be network locked as you suspect. Does the mobile reads your T-Mobile SIM card? If not it has chances to be network locked. You can unlock a Samsung E760 safer and easier using unlock code. You can get the unlock code for a Samsung E760 from www.TheUnlockArena.Com
at affordable cost with free unlocking instructions.
Things to not before you unlock * Only GSM mobile phones can be unlocked to use with your T-Mobile SIM card. * You must find out the network, county, IMEI number of the network unlocked phone before unlocking.