Echo on land line incoming calls only?
Is it ALL land line incoming calls?
Absolutely no cell calls echo?
Is it one number, a small area [neighborhood? city?], or across a bunch of places?
Does it happen when you call them?
Are they using cordless or corded phones?
Are you 100% certain it's a LAND [copper] line and not a VOIP line [Vonage or the like]?
I ask these questions because it may not be your side of the coin, rather the caller's side.
The only time I've had echos in any of my cell calls [whether they were T-Mobile or previously Sprint] was when I had a bad connection, and it was the other party that was generally the problem [super-compressed digitization on their end, their phone turned up all the way so it echos, etc].
Yeah -- Wifi would be VOIP and VOIP generally has echos.
So, deardav1 is on the right track. Unfortunately on the wrong side.
I'd talk with the other people to see if the AT&T lines are UVerse or copper. And I'd also see if you can do tests from true copper lines to see if you get the same echo.
I'd almost wager any echo call you get is VOIP in some way -- whether it be on the T-Mobile end (if it's WiFi calling) or on the other end (UVerse, Comcast phone, Charter phone, MagicJack, etc)