Google Voice allows callers to forward calls to their Google Voice number to any other phones simultaneously until one picks up. Therefore someone could set their home, mobile, and work phones all to ring when someone calls. Constant availability with one number including SMS and it is free. Google Voice provides a web interface and an Android Application for mobile phones to change settings. It can be connected with Gizmo5, a Skype alternative, to allow one to send and receive free US phone calls or make cheap international calls.
Google voice will maintain a permanent record of voicemails and will transcribe them and email them along with the audio to the owner. The option exists to receive SMS of the transcription as well.
We have been using Google Voice for over a year and it is wonderful. It has become our primary phone number. All other phones have become access points to the Google Voice interface.
GV is brilliant, unparalleled and free.