Referring to the ability to port a number across carriers, i.e., move a telephone number from one carrier to another in a competitive environment.The United States first required number portability, initially with respect to toll-free numbers. The Telecommunications Act of 1996 required local number portability (LNP). In 2003, the Federal Communications Commission required wireless number portability (WNP), extending portability to cellular telephone numbers. Number portability is possible in many countries with respect to toll-free numbers and landlines, in general. Portability is possible in fewer countries with respect to cellular service.There are restrictions, however. For example, it generally is not possible to port a number across landline and cellular domains. Neither can numbers be ported across countries. See also LNP.