You probably want a phone that has a good battery life, clear phone calls, and great reception. AND, probably something cheap to save you some $$$!
I'd go with an older Bold, although I own a Storm(which also works good as a phone phone). Although I'm not the most knowledgeable, I hear nothing but good things from Bold 9000 and and 9700 as phones.
Almost all BB phones are great phones just for voice calls, so you'd do great if you got any BB. I'd say get the cheapest Bold you can find. Hope this helps.