If you are unsure of the warranty status or the network the iphone 2g 3g 3gs or iphone 4 was sold on.

Factory unlock code for iphones where unlock via itunes can be done need the info of the network.

Example Log from Service.

Checking with server...

Verifying

Processing...

Info:Requesting info

iPhone 4 16GB Black (4.2.1)

IMEI: 01xxxxxxxxxxxxx  

Limited Warranty:Serial number: 7W0xxxxxx

2011-01-16 / 2012-01-15

Sold to country: United Kingdom

Carrier: Orange GBR

Now you will know the network the iphone is locked on.