Do I need to purchase SIP proxy server separately?
You do not need to purchase a SIP proxy server separately from your Brekeke PBX license. The SIP server for PBX (SIP proxy server) is bundled with Brekeke PBX.
There are some difference between stand-alone Brekeke SIP Server and bundled SIP server for PBX, you can compare supported feature for each edition from the link below:
http://www.brekeke.com/sip/feature-list.php