while I appreciate coz's enthusiasm for grandcentral, it has been closed for some time, and I'm not sure if it will ever be open again as a free service. I do however think you may as well "reserve" a number, if nothing more than s&g. I have been looking at alternatives for this and there are many.
personally I was looking for an all-in-one with 800#(I have a vanity #), exchange hosting, voicemail2email, and eFax. I currently use ringcentral for my eFax, but they don't have exchange. the best option I've found so far is OneBox, they claim direct push exchange, as well as pst import on their executive plan ($17/mo).
http://www.onebox.com/products/exec-feat.asp
The problem lies with the cell carriers, as they often charge to fwd to an offsite #.