Moving Countries and Changing Sim Card. Iphone is locked by ATT and Not Sure How to Unlock the Device for My New Sim.

I moved to Switzerland for school and will be residing here for the next 2 years give or take. I purchased a Sim card from Swisscom and when they inserted it into my phone today there was an error message. I forget what it said but it said to contact ATT to unlock the device or something. After researching for a bit I found this forum page where someone had a similar issue and person said to check your General->About in setting and see the Carrier Lock. 

So long story short my question is how do I unlock the carrier settings and who should I contact? Do I need to disable my service? Because I will be returning to the US for vacations and holidays and would like to use my ATT sim when in the country.