While working for a telecommunications company, I was able to dive ever so slightly into VOIP.
I did the DOCSIS side of the configuration for the devices that operated VOIP across the Cable Network, configured the necessary mibs, and set up InnoMedia devices and Adtrans primarily. There were a few PBX's that were configured and set up for a variety of tasks. Base familiarity is present in concept. However, it has now been well over a year since I have done any of these tasks.
Session Border Controller (SBC)
Inno-Media
Adtran
IP Phones
SBC
AOS
FreePBX
SIP