Cell phones: AT&T Selection: Best Reception in Rural Areas (WA)

AT&T Selection: Best Reception in Rural Areas (WA)

I am trying to upgrade a phone for my mother on my phone plan. She lives in a semi-rural area in Western Washington (about 25 miles south of Tacoma). We both currently use a Sony Ericsson z525a but she only receives spotty service at her house. Her friend who has an LG that is a few years old, can actually make and receive calls at her house, while the Sony can only send/receive text messages and voicemail. On a good clear day if you contort yourself just right in the window, you can call out, for about 5 seconds. This is literally the edge of the service area and could also be due to many large fir trees as the phones work great about 100ft down the street.

What I need to know is which manufacturers typically have the strongest signal reception? What are the best options out of AT&T's current lineup? We will probably switch in a month. I plan on getting a Blackberry Pearl, she'll probably take anything that's free to about $25. She's a cell phone minimalist, uses about 30 minutes a month on the bill, no cameras, wants biger buttons and a easy to read screen. I live in the city and don't have a land line. I just want to make sure this time around she can make the most of her cell phone and possibly ditch her overpriced land line too.