As of 2015, usually only prepaid mobile phones are sold with a SIM lock. Phones sold with a contract stipulating monthly payments are not typically locked (as the monthly payments are due no matter what network the phone is used on). Also, most providers will unlock the phone on demand.
Regarding this, are all phones unlocked?
Verizon uses the CDMA network in the United States, but many of its devices also include an unlocked SIM slot. This means most Verizon phones — including all LTE devices — aren't locked. Unlike Sprint, the SIM slot on Verizon iPhones comes unlocked.
Are prepaid AT&T phones unlocked?
Determine if your AT&T cell phone is CMDA or GSM. CMDA phones cannot be unlocked, while GSM phones can. By unlocking a phone, all you have to do is switch the SIM card to change service. You can see why it's tough (if not impossible) to unlock CMDA phones.