SIMs are always used on GSM phones; for CDMA phones, they are needed only for LTE-capable handsets. SIM cards can also be used in satellite phones, smart watches, computers, or cameras.The first SIM cards were the size of credit and bank cards; sizes were reduced several times over the years, usually keeping electrical contacts the same, so that a larger card could be cut down to a smaller size.SIMs are transferable between different mobile devices by removing the card itself. eSIM is replacing physical SIM cards in some domains, including cellular telephony. eSIM uses a software-based SIM embedded into an unremovable eUICC.