Someone recently mentioned to me that, as a new poster asking about cellphone options for travel, no one ever told them that a new SIM card will give you a new phone number. While those of us who deal with SIMs are aware of this, a lot of people new to the whole idea might not be aware at all, and have been caught out when they could not be contacted on their original number after switching SIMs. I thought maybe a little intro to SIMs might help some people.
A Subscriber Identity Module or Subscriber Identification Module (SIM) is an integrated circuit chip that is intended to securely store the international mobile subscriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile devices. They are designed to be transferable between devices. It is the "brain" of the phone, giving whatever phone it is installed in its own identity (phone number). Every GSM phone has a SIM. CDMA phones do not, although newer LTE phones that usually operate on CDMA systems (like Verizon) can also utilize SIMs to be able to access GSM networks abroad. In the past, phones were routinely "locked" to their home company, meaning that you could not switch an AT&T SIM for a French SIM without first getting permission from AT&T to "unlock" the phone. If the phone was unlocked, the original SIM could be switched for another, then replaced when arriving home. Anymore, most newer phones come already unlocked, so you only have to change out the SIM.
SIMs come in different physical sizes, depending on what type your phone uses. Full-size and Mini SIMs usually come together on the card you buy at the phone place, and they install whichever one you need. Micro or Nano SIMs might be a bit more difficult to find. SIMs that you buy abroad generally come with some minutes already on them, and you will need to ask how to add more, if necessary, later on. These SIMs expire after a period of time, so even if you don't use up all of your minutes, unless you travel frequently enough, they won't still be good the next time. Each new SIM comes with a new phone number. You don't generally have to have a different SIM for each different country you are in, With a few exceptions, you can "roam" on whatever SIM you have (although at a slightly higher cost when outside the country where you bought it).
"Travel" SIMs, like Eurobuzz or Mobal, come with their own phone number, too, but it is permanent. You aren't charged for anything until you make calls. In between trips, even years apart, you can just put them away until next time. The downside is that they are a bit costlier per-minute to use. Some of the travel SIM firms have different methods of making calls, so you should ask questions about that before deciding which SIM to purchase.
We can go on for days about all of the details (and we often have), without mentioning something as basic as the number change, or that SIMs expire. Hope this helps some who might be new to international travel with a cellphone!