I’m on a (long out of contract) cell service plan with Sprint. I want to switch this number to a voicemail-only service (people should be able to call the number and leave voicemail, but I don’t need any other features and don’t need a phone to be able to call from that number) for the lowest possible monthly cost. Should I port it to a prepaid phone? (I happen to own a Virgin Mobile phone that’s not being used, but I’m not attached to using Virgin if there is a service that will be cheaper.)