Old ATT sim card question...

I have had an account with ATT since before they were cingular and I want to get a new phone for the account. I do not want to get a new sim card though, because I will have to sign a new two year deal with AT&T, at least that is what they told me at the store. My current month to month deal is like 29 a month and we get 400 anytime with unlimited nights/weekends. If I just want to get a new phone and still use the old sim card, what should I look for on eBay or with some other retailer?

I also have another problem, my work phone is through ATT and I do have a new sim card with that one. I have been using an MPX220 with that account, but the phone just died. I need to get a new phone, but am unsure which to buy. My work does not pay for much in extra features, just all the calls I need to make and international calls also, since I work down in Mexico alot. However, I do sync my phone with outlook (just contacts, to do lists, and calender - I do not use the email part at all because my work won't pay for it). I need to get the cheapest phone that will accomplish these functions. Can anyone advise on this?

Thanks everyone in advance!

Slim

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Thanks so much for the reply to my post Ellen! My main concern is that since I have the old ATT sim card from before they got bought by Cingular, that I would have to be careful which phone I bought. That for instance, if I bought a new Blackjack II, my old card would not work in it. So I was thinking I might have to troll through eBay for an older phone for which my card would work. But your saying any unlocked phone will work?