Sim cards are universal (for the most part) but the phones are not. New phones require a 64k Sim while a lot of older phones make do with a 32k sim and a lot of new capabilities and features are requiring the higher 64k model,SIM cards are a feature of the GSM standard

SIM cards are good for consumers in that they  are also available for international use.

If you will are traveling to several differant countries, the  rates may be higher if you have to “roam” outside of one calling area.  SIM cards are usually available prepaid or on a contractual basis  abroad.