VoIP PBXs are legal, I have personally deployed several with the knowledge of POTRAZ. If you intend on deploying your voip PBX within a building for internal communications amongst employees there are no restrictions on that.The restrictions arise when you want to use your voip PBX to link up 2 or more offices in different locations eg: office A in eastlea and office B in Newlands.
In fact,in the previous example the restriction is not on the VoIP pbx but on the use of your IP infrastructure (eg VPN) to carry voice. A simple analogy would be that of blackberry phones and services.POTRAZ does not put a restriction on the use of Blackberry phones but it has restrictions on the use of Blackberry services.