I've been a big fan and user of GrandCentral for quite some time. I know there are other services that are quite similar. Here's one I just discovered that may be worth investigating further.

I'll be talking to these folks soon about a piece on fixed mobile convergence I'm writing and will share what I learn here as well.

QuesCom offers a unique solution for ONE-NUMBER provided these following features.

• One-number: fixed, mobile & fax - The QuesCom user receives all incoming calls and faxes onto a direct Unique Number. Indeed, for convergence purposes, all vocal, fax and SMS messages are now centrally managed in a single mailbox.
• A single number called, all phones ring - Any call incoming onto the user’s One-Number makes ring all the phone devices simultaneously (until 5 devices!). When the user takes the call from one phone, the remaining phones stop ringing.
• Seamless voice call continuity - Without interrupting the call, the user can switch from one device to another, mobile or landline. A call that has been started on the mobile phone can be continued on the landline phone. This call toggle (manual or automatic) is transparent to the user.

QuesCom is a leading player on the markets of VoIP-mobile convergence and Telecom application solutions and a member of Nokia PRO Forum, Nokia's advanced developer program. The company has more than 100 European resellers and is a certified member of major PBX vendor programs as 3COM, Aastra Matra, Alcatel, Avaya, Cisco, Innovaphone, Mitel & Swyx.

http://www.quescom.com/mobility