The ShoreTel Mobility Routers (aka SMR) is an exciting product that can make fixed/mobile convergence a reality!  It builds heavily on inherent SIP functionality in call setup, call flow and Session Description Protocol.   It acts as both a SIP client and as a SIP proxy server (e.g. B2BUA).   If you are comfortable with SIP protocol, the SMR is a relatively simple device to understand.   The products primary contribution to the state of the art is the ShoreTel Roam Anywhere Client (aka SRAC) and the fact that it is available now;  it works and there are plenty of reference accounts. The product demands a pervasive, voice enabled wireless network with a best practice recommendation of controller based access points.    In my opiniion the product is optimized for Campus environments in which the same QOS requirements that you would expect for WAN connectivity are strictly adhered to.  The Wireless  environment requirements are for advanced network and power management strategies.   The product is oriented toward a CISCO like Wireless networks “best practice” deployment.  Clearly, you can make VoIP calls from StarBucks but you will not have the QOS that you would have on an enterprise Wireless Network. The product can be integrated with most any PBX that supports SIP integraton including Microsoft Linx.   When implemented as part of a  ShoreTel deployment there are license requirements  for both the SMR and the ShoreTel iPBX.  For example, the SMR interconnects to the ShoreTel with both SIP extensions (Wifi connections) and Sip Trunks (Cellular connections).   The number of paths, as you would assume, is calculated based on simultaneous conversation estimates.  You are required to have two different ShoreTel PBX extensions if you are Mobility user.  (My guess is the ShoreTel "twinning feature "was developed in part, primarily to support the SMR).   It is not clear if the required SIP trunk,  SIP extension and ShoreTel User licenses are bundled in the SMR acquisition cost but they are required.  There are also extra SMR licenses required for presence and secure voice. The best working model you might use for discussion purposes, is to envision a “tie-line” configuration between two pbx systems.   In this case the SMR acts as a tie line between the iPBX and the Cellular network.  You do a 10 – 4 Digit translation on the Cellular side of the tie line to reach ShoreTel extensions; and you do a 4 – 10 Digit translation  on the PBX side of the tie line to reach Mobile phones.   The SMR acts as a registration point and based on the SRAC calculations of WAP signal strength and active call can be handed off to the cellular network. An incoming call to a ShoreTel users deskphone, also rings the associated SIP extension via the SMR.  The Wireless SIP extension is the ShoreTel Roam Anywhere Client running on your faviorite smart phone.  The SRAC is smart enought to register with the SMR either wirelessly or over the Cellular network depending on signal strenght.   A call from the SIP extension displays the CID of the ShoreTel deskphone.  The product is fully formed and is an exciting addition to the ShoreTel product family.  Another Brilliantly Simple Solution!