Activating an iPhone 5s with MVNO

If I would buy an iPhone 5s locked to ATT, would I be able to activate it to use with an MVNO such as Airvoice wireless? Could I buy a brand new iPhone 5s and insert an Airvoice SIM in place of the ATT SIM inside, and still be able to activate and use the phone normally, since Airvoice uses ATT's network? Or, would this only work with unlocked iPhones? I'm new to iPhones, so I'm not sure.