I am nearly sure that someone has already done it, if not I will.
But have you done for Skype and/or an Ip Softphones like X-lite or why not X-meeting .
I am using Skype for the internal communication in our organisation.
and X-lite with a SIP ( Free France connection). ( ok for Fix line).
it works fine.
and I am looking now to find a solution to call on mobile directly from the labtop from mobile to mobile. ( and the communication pb cost will be ok).
I could imagine that Zimbra would be a good solution to be the reference database of phone/email/IM database.
(I can see many implementation of Zimlet that will give a great value to Zimbra solution).