its called LNP-or local number portability-its a federal regulation that allows you to port your number from one CLEC or ILEC ( local exchange carrier) to another-it also applies to cell carriers-i am a verizon customer but have a number that is technically owned by AT and T. Is should extend to a VOIP carrier like Vonage-however im not sure about magic jack since i believe they sell you the device rather than the service as a carrier.