You do not need two gateways. The way that most people do, they get a gateway for the site in South America. They collect the calls, do the billing and pay a service provider in the U.S. to handle the calls.
What you need:
1) One gateway, as many lines as you need. If you could afford the price of the local phone company, go E1 service (30 channels). If you need to start small, something like 8 analog lines will do.
2)Internet bandwith/access. Get at least 15-20 kb per call that you are planning to pass trought.
Static I.P. address are not mandatory, but will help a lot. Try to get static if you could afford it.
3)A billing system with IVR. That will be the mostr expensive and hard to find part. Many crooks out there pedling ilegal copies of somebody else software. Be aware!!!!!
4)Local phone lines. Analog or digital depending of the gateway selected. Try to go digital if posible
5)A provider that will give you the handling of the calls on the U.S. Many around.
The shorcut: Some providers will give you the billing and IVR if you send them the calls. You do not make as much money, but you could save yourself a lot of troubles. In that case, your gateway receives the calls from PSTN, converts them to VOIP and you get all the prompts and billing from the provider.