The FCC has a nice little FAQ about unlocking cell phones.

Yes and no. There are 2 competing 'guidelines' which cause some issue...one being vague wording in US law and the other being individual carriers' definition of a current customer (or 'customer in good standing')...

Back on August 1, 2014, President Obama signed the "Unlocking Consumer Choice and Wireless Competition Act" into law. That became effective on February 11, 2015. Essentially the bill says that carriers are required to unlock devices which are fully owned by the user and when the postpaid user is a 'customer in good standing' with the carrier.

So, basically, the phone must be completely paid off (purchased for full price or installment financing payment plan completed). Also, the user must be a postpaid 'customer in good standing' with the carrier, which means no outstanding debt (unpaid bills), contract completed or Early Termination Fee paid (or for non-contract carriers/plans, the customer must be 'established' with the carrier).

That last bit is the sticking point. As more and more the carriers move away from contracts for postpaid customers, it is basically left up to the carrier to decide what an established customer is and how they define it.

All of this is assuming you buy the phone directly from the carrier or with carrier incentive via 3rd party seller.

However, the bill signed by President Obama left a little gray area...what if you buy the phone for full price from the manufacturer directly at full price, with no carrier involved? Theoretically, you would meet the guidelines for the phone to be unlocked at the time of purchase (or sold as unlocked). But since the bill specifically speaks about carrier unlocking of devices, there is a question about manufacturer unlocking.

Thankfully, Apple and Google eliminated the gray area by ensuring their direct-sale devices are factory unlocked before you buy them for full price at stores or online. I think buying a Windows phone for full price directly from the Microsoft store is the same way, but not 100% sure.

As for prepaid customers...the carriers are only required to unlock the device by the deadline stated as '1 year from the date of initial activation date' or immediately (which is about 48-hours for most carriers) if the customer has been with the carrier for more than a year. I believe T-Mobile only requires a total of $100 in reloads before unlocking the phone in lieu of a minimum time commitment.

I believe Sprint and Verizon only use/need SIM cards for the GSM/LTE radios in their devices. The CDMA potion doesn't use a SIM.

However, back in 2012 (according to this article), through a spectrum swap & purchase, Verizon was required to unlock the GSM/LTE side of all of their phones. To this day, all phones purchased directly from Verizon are GSM unlocked right out of the box. (Here is a link to the text of the 700 MHz Block C access requirements - see section (e) for guidelines on not locking phones.)

However, some Verizon versions of phones have "software bugs" which make it difficult to use the phone on a US GSM carrier (T-Mobile, AT&T, etc). Its widely speculated that Verizon has done this on purpose via bloatware on most Android devices (see this link).