replacement phone question

[peter1]

hi - newbie here and need some advice although I'm sure this has been asked a million times i couldn't find it.

my wife's at&t iphone 3g has become unreliable. the contract has expired but she doesn't want to replace it until the new ones are available. can i just buy a used 3g somewhere, drop in her sim card and have it work until the new ones are available?

if not, what would you all recommend.

thanks