We did this a while back - but since we make and sell software which enables Fixed to Mobile Substitution (FMS/F2M), we are of course eating our own dog food.
Our receptionist uses MobilePBX which is a software PBX application connected to a mobile phone on her desk to receive and transfer calls. With integrated real-time mobile availability status for all of our mobiles as well as calendar integration, she can easily see whether the intended recipient of a call is available or not - before she transfers a call.
Our IVR (voice response) software handles our main company number and transfers to the correct department depending on the caller's choices.
When we are at our computers, the rest of us use a CTI application which is connected to our mobiles using either cable or Bluetooth, so we can control the mobile from the PC - including dialing contacts straight from CRM, Outlook, and more.
If you should be interested, check out http://www.mobilecti.dk/.