Gifted replacement phone iPhone 4

a friend of mine had a replacement iphone 4 coming his way, but he had already got another phone so he gave me the iphone as a gift. the phone was unlocked so it could be put on any carrier but when i brought it in to get it hooked up it rejected the new sim card, and the lady with the carrier said something about it only accepting my friends sim card or something. What is on the go here, andnis there any way to fix it?