Ask HN: What's the cheapest way to keep a phone number and forward calls?

I have an old business phone number which I want to keep in case somebody calls. This will happen maybe once a month or less. Certainly not worth keeping a full VOIP plan for it. I thought I could port the number to Google Voice but this seems to work only for mobile numbers. Does anybody know about a cheap service that allows me to port the number and forward calls to a new number?