Thank you for the advice. My APN setting is accurate, but where is the "message center number"? I didn't find it in APN or Message app settings.
I soft restarted each phone and mms seems to be working now with limit of mms < 1 MB, ie can't send typical camera higher mag photos.
Found signal is generally weak and unstable, much more spotty than ATT coverage map. I almost never had this issue with Verizon. I know Verizon phones don't cover all ATT freq bands but not sure this is the reason for my spotty signal.
