When you move your SIM, you keep the same cell phone service. SIM cards make it easy for you to have multiple phones so you can switch between them whenever you like. These phones have to be either provided by your cell phone provider or they have to be unlocked phones.  With unlocked phones, you can always switch your phone service between different phones as easily as popping out the SIM card and moving it.  You can also use different phone service plans in a single unlocked phone, say one for work and one for personal, by switching the SIMs.  The opposite of an unlocked phone is a locked phone.  In contrast, only SIM cards from a specific cell phone company will work in its locked phones. These phones were locked to protect subsidies, i.e. the amount that your cell phone operator subsidized to have you sign a contract with early termination fees.