Find the best rate for calls to Cuba from your PSTN provider.Negotiate a deal by volume.
Find a internet service provider that could sell you bandwith to the internet, leased line or radio link. You will need 12 to 15 kb of internet for each channel of PSTN.
ON the PSTN or long distance provider, stay digital. Ask for E1 or fractional, but not analogs if you want to keep your hair.
After that you need a gateway, that will do the conversion from internet VOIP to PSTN.
Do not need a billing system ,since you will be selling “bulk”.
Contact a brokerage group, like IPCB.com where you coulf offer your minutes to others.
Make money, be happy.