This is more convenient than you might think. At least you don't have to carry two devices on the road. Nor are you tied to a smartphone with two SIM slots or, for example, an E-SIM. It works with any standard SIM card - whether you're using a Nokia8010 from 2005 or the latest iPhone. It works anytime, anywhere, even abroad. The SIM card connects to the telephone exchange, so you can be reached or called on multiple numbers fixed and/or mobile - and even time-based!