The one problem you might run into is what if the cell service goes down, rare but does happen. Is your ATT voip?
If not why not get a $30/mth Voip line from your cable co?
Do you use a voip? I have looked into that but most complaints seem to be sound quality issues.
First I dumped landline and went VOIP, then I dumped VOIP, haven't had any problems with cell only. Just make sure you get good reception at home.
You can get ripped off with your cable company's VOIP, as you basically pay extra for the convenience of using them for everything, so look at a cheaper service like Skype or Vonage.