Sprint does not use SIM cards in any mobile phones. In lieu of SIM cards, Sprint uses MSL technology to unlock a phone. SIM card-enabled phones do not require unlocking and are universal with one another.
There are two different types of mobile phones: CDMA and GSM phones. Most carriers, including Sprint, use CDMA technology, which does not include SIM cards. As of November 2014, the only large carriers in the United States that still use GSM technology with SIM cards are T-Mobile and AT&T. This makes it possible to seamlessly switch a SIM card from an AT&T phone into a T-Mobile phone.