My first piece of advice for US users would be that if you port the number in you have a better chance at porting the number out. Even JFax/eFax lets you keep a number your brought with you. Second, many VOIP providers let you take a number and use it as an inbound fax number. It is a flip on/flip off service. This puts you in a better position to fight the argument that JFax makes which is that the special category of service they offer is not covered by number portability rules. Your ability to see that you also take calls on it occasionally and make outbound calls on it undercuts the argument.

Callcentric's numbers can be usesd as a virtual fax number.