It situations like this you need to do some elementary detective work.
1) Change the channel that your ATT cordless phones are using. If you don't know how to do that, read the manual. If you don't have the manual, it will be on their site (note that ATT phones are NOT made by ATT, they sold the name off years ago).
2) Position the base station away from routers, etc. that may be interfering.
3) Remove the ATT base station (unplug it). Try a different phone.