I recently left ATT (2 months ago) and went to Metro PCS. I bought a LG F3 from Metro PCS. The service in my area is not that good and i wanted to transfer back to ATT. I did not have a smart phone from ATT so i wanted to know if i could bring my F3 to ATT
If you get the phone unlocked before you leave MetroPCS, you could use it on AT&T. You won't get LTE service on it in many places, but you should get 2G (for voice and text) and HSPA+ for data. This is assuming that you have the model that is shown in the link below.
What 21stNow said is likely correct. But you should verify that your phone is in fact a model that supports HSPA (3G) 850MHz and 1900 MHz if 3G data is a deal breaker for you. If your phone does not support these 3G bands, it won't work on AT&T's data network, and you will be stuck with EDGE (and WiFi, where available).