Essentially this is a broad outline of how it would work. You will need a local PSTN (tel #) access number in the US to which your clients will dial. Calls to this number will hit your US gateway, so you will require an IP address, internet access and appropriate bandwidth to handle 20 concurrent calls. Your US gateway then sends the call over the internet to Guianas, this is where Satellite internet becomes relevant, as I presume the local telecoms infrastructure cant handle your requirements. You will require a local gateway, an IP address, internet access and local PSTN number for your Guianas gateway to dial the destination through, You have to bill your clients somehow so will need a billing system.
So in a nutshell you will require the following:
Gateways x 2
IP addresses x2
Internet access (US and Guianas)
PSTN access (US and Guianas)
Now be advised, satellite access doesnt come cheap