Any valuable suggestion would be much appreciated.
It's too late to start VoIP business in Poland - there are many companies on the market already. Another thing is that many ISPs offers VoIP as addional service ... or even they offer internet+voip+cable tv.
how much is the average cost of dedicated (leased) Internet broadband line of speed say 512Kbs per month
~15-20$ (local ISPs)
does the ISP companies allow voice port or they block it etc.
there is no such thing as VoIP port.... VoIP isn't easy to block, so I can say that in most cases there won't be any problems to establish VoIP connection.