how do i unlock an iPad 4 that seems to be locked to a wireless carrier I do not use

I purchased an iPad 4 in April 2013 and also purchased the extended warranty. I had several problems with the system crashing and sought help (three times) from Apple. The most recent time I took the iPad to Apple, the Apple Genius told me that the machine would continue doing this...that there was not a long-term solution. Noticing that I had the extended warranty he offered to give me a brand new iPad 4. I just sold the new iPad and the purchaser is unable to link it to the wireless company she uses; she said that the system was "locked" by another carrier. This doesn't make sense if I actually received a new iPad in exchange for the old one, and I'm trying to see if anyone has had a similar problem (locking) and if so does s/he know how to "unlock" the iPad. Thanks for any help you can give.