Where will you be using your AT&T phone the most? Have you heard of the AT&T Microcell device (
http://www.att.com/standalone/3gmicr...id=cqGOqkFV1Fz)? Just plug it into your cable modem or router, and it will provide a great cell phone signal across an area ~5000sf. This allows you to use your cell phone at home, at your office, or whereever you place the device.
Like others around the lake, I have no cell phone service at my house. The Microcell has been great. I set it up in a corner, and it covers the whole house, upstairs - downstairs - every room - and the porch. I recommend you look into it.