A multi-carrier GSM phone with two, three or four SIM cards. Dual-SIM phones are popular with global travelers who use local and foreign carriers as well as people who want a business and personal line in one device. Triple SIM and quad SIM phones support three and four carriers respectively. Following is the alphabet soup. See SIM card
DSSS (Dual SIM Single Standby)
Older dual SIM phones had one transceiver and only one carrier would be active.
DSDS (Dual SIM Dual Standby)
Also called "DSS" (Dual SIM Standby). Two receivers/one transmitter. Accepts calls from one carrier at a time.
DSDA (Dual SIM Dual Active)
Two transceivers enable calls with two carriers simultaneously.
TSTS (Triple SIM Triple Standby)
Three receivers/one transmitter. Accepts calls from one carrier at a time.
QSQS (Quad SIM Quad Standby)
Four receivers/one transmitter accept calls from one carrier at a time.