What do I need for PBX gateway

I use to own a dialup internet service and I would buy blocks of ports to resale dialup, My provider is now in the voip business and they have POPs in all major cities they say they can provide me with unlimited lines at no cost all i pay is munites very cheap so I can resale they said I need a gateway what they are asking for is an public IP to route all traffic to and then I forward to my business cutomers PBX how does that work and what do I need?

Armando Muniz