One of the many Google services, Google Voice provides a free telephone number which can be modified through account settings to either forward calls to another number (or multiple numbers), or e-mail you when a message is left in the free associated voicemail box. The service allows people who are very mobile and changing locations to have a number that can be used their whole lives due to the ease of forwarding. The service is ideal for the homeless, as messages can be checked online just as easily as via owning a physical phone, which isn't necessary.
Currently the service is in limited availability, and an invitation must be requested using the link above. After the invitation is received, then user can pick any available phone number from many different area codes, and begin setting up the account settings and phone and e-mail forwarding service. A physical phone is needed during the set up, but can be turned off after set up if the user wishes to use the message service only.