I have a 431 that I use as a backup phone when working outside. I've been through two of these and both go to no signal frequently. No other att phones do this that I've had so I know it's the phones. I'm not talking low signal I mean none like you are in airplane mode or similar. Just curious no big issue since its a backup phone.
I am on the cricket net work and my first phone that still have is the zte engage and have the best signal with it I liked it so much that I bought the zte source about the same size as the samsung galaxy s 3 and about the same specs I would much rather use use zte over samsung .could it be your location?