I have my pc's , router ,dsl modem, ATA, and cordless phone base all plugged in back up power supplies. My pc's will shut off after 5 minutes with the software provided. With the pc's on battery wise my phone service should stay active for at least 45 minutes according to the software.

Now take in to account the fact the pc's will shut off if there is no power after 5 minutes and the amount of time probably doubles.

AT&T's hardware has batteries so it should run for awhile. If you do not have your pc on a backup power supply I strongly recommend them. They will make all your hardware last longer. They also kick in and regulate the power if it goes above a certain level aka surges or drops below a certain level.