The installer wants $15k
VoIP may be an option, what's the company?
Any authorized dealer will charge a high markup. Google one of the model numners to find a few websites, and then PUTPAC that website's sales. IME, you could probably save 5k. Be very careful with the T&C for the warranty. Make sure the installer (or some other IT/pbx) is guiding you. The list you gave is very specific.
VOIP can mean two things.
1. What most people think of. The actual phone call is serviced through the internet. GV, Magicjack, Ooma, etc are all consumer level examples. And there are commercial products as well.
2. VOIP can also mean the way the phones communicate with each other and the pbx. Meaning that the transfer of data is over IP (to exclude analog oe digital). The actual phone calls are still POTS (Plain Old Telephone Service).
I did not look up those part numbers, but from the descriptions it sounds like it is a VOIP PBX with POTS (2 above). The need for a 66 block would indicate POTS.