what a great problem to have, congratulations! What I would suggest is a hosted PBX service. There are quite a few of them out there, some of them are focused on the real estate agent, and some of them are a little bit more generic.
With a hosted PBX, you'll be able to route incoming calls to your cell phones or to voicemail boxes depending on the option pressed by the caller. The beauty of the system is extended and that he can also keep detailed call logs and use them for lead follow up.