ATT plan-Must I use an ATT smart phone?

My wife and I are piggybacked on a plan with our daughter with ATT. Our current cell phones are the old flip top.
I am considering going to a smart phone.
When I went to ATT the smart phones were more expensive that I am willing to buy. The rep, in answer to my question, said I could get a phone from WalMart or Best Buy, but it would have to be an ATT phone.
However, when I went to BB the rep said I could use any phone, and he would put my SIM card in it, and it would work OK. It did not have to be an ATT phone.

I do not understand the plans and wonder if the BB rep is correct? Instead of a several hundred dollar smart phone I could get a phone by ASUS for slightly over $50 on sale.

Anyone with knowledge or experience in this area? Some help would be appreciated. If I have to use the expensive phones that ATT had I am going to forget it.

Thanks,
Jerry

 

@JerryM You will be able to use any unlocked phone as long as it supports the same frequencies that AT&T uses.

 

Thanks, Roger. I guess I might have to get a new sim card, but I think they are less than $10. I will check and see if mine will fit.
Regards,
Jerry

 

Check Amazon. BB is ok. Avoid Walmart.

 

Thanks, Jack,
I have decided that I don't really have any use for a smart phone, but I appreciate the advice I have received. It will be useful if I change my mind.
Regards,
Jerry