Maybe yu already know this. I didn't and now I have an old cell phone that is almost worthless. So, if you are thinking about using an older model cell phone, check first to see if it is compatible with current 3G an4G networks.
AT&T did make announcements.
I missed it. 
Watch out! At some point present 3G and 4G networks will stop.
2G and 3G to be phased out by 2020.
Mobile operators could close legacy networks in five years.
Mobile broadband customers will no longer be able to use 2G and 3G platforms by 2020, as 4G and 5G become the industry standard, analysts have predicted.Apr 30, 2015
https://www.uswitch.com/broadband/news/2015/04/2g_and_3g_to_be_phased_out_by_2020/What they do not tell yu is that is some areas 4G is not usable for data. You have to have 4 G LTE.
