Faxing can be a little less reliable over these type of lines then with POTS. If faxing is important, or if you send or receive large fax documents I would look into leaving 1 line as POTS (if possible), or look at an e-fax solutions.

As mentioned, PRI is a better hand-off to the CIX. I also like SIP trunks over analog, but that will require some upgrading of your equipment, and ATT may not even offer this.