After far too much frustration and procrastination, I've got the following done:
-the mailbox service is built and deployed along with SIP-Communicator. Tricky, but the online tutorials proved very helpful
-the mailbox service now listens for an incoming call. I had it wait 10 seconds and then print out to the console, so the first milestone is complete. Sort of. Right now, when the mailbox service is informed about an incoming call, it loops until either the call is picked up or 10 seconds have elapsed. the problem with this is that the flow of execution is trapped within that method, so you can't actually pick up the call ;)
Problems to solve for next time - how can I wait for an incoming call while allowing the rest of the application to continue normally.
Next Milestone: connect the call and play a file.