Where exactly is the ESN built in on an IPhone 4 Verizon ?

I have a Verizon IPhone 4 16GB with a Bad ESN, the home button stopped working on this iphone so I took it to the Apple Store and they gave me an exchange, meaning another brand new phone, I don't think they knew it had a bad ESN however. ....now since the last phone had a BAD ESN do you think this new IPhone I received from Apple will also have a bad ESN or it should be clean ? Please let me know..thank You. !!