My first job was to tailor a FreePBX system to our users' "needs". Management decided that our sales team's "needs" are to get email reminders after each client call to keep the SalesForce log up to date.
The sales team did not agree with that, but it was fun to build anyway!
What linux to unix is what FreePBX TO PBX. Basically think it as an open source google voice. Any one interested in SMS, VoIP then this the project to take a look.
It's basically a pre configured Asterisk server with very, very many gui setings and lots of paid extensions.
That can basically do anything you can think of and then more.