The Unified CallConnector client/server offer presence functionality along with other features for CME. Take a look at the following link.
Hope this helps.
Cisco UCC price list
Client license (per user) $65
Server license $2,595
Mobility license $1,995
Server requirements: Small sites up to 25 users; dedicated 1.8-GHz or faster Pentium 4 or compatible processor(Virtualization not mentioned).
Time to install, configure, work around bugs and train the user: 1 week maybe.
All that to dial from a PC and see when others are on a call. Both things are doable spending a fraction of the above.
I have found that these are the kind of things that make dificult to sell such add-ons.