no one should get charged for the "number". think of it as an account number assigned to your airvana.... thats all it is.
some people ARE being charged for the unit. Lets say I live in a good sprint coverage area but in my basement, i get poor coverage. so i can lease this equipment to get better coverage there.
some people dont have good coverage AT ALL. sprint confirmed this one way or the other and they offered this service free of charge.