Linking two phone lines..?

i live in mexico and i make dial-up connections to the us to connect to other computers because of my business, but making long distance calls to the us is really expensive, i was trying to use a modem over an ip phone line but it seems that no company supports this yet, so what i want to do is get a trunk line to the us so all i have to do is dial an extension number and a phone will ring in a office i have in the us, but here is where it gets tricky, i want to lease a phone line in the us ande i want to link the trunk line with the phone line i have in the us and make the line in the us make the dial-up call and connect to a computer via modem from my computer in mexico, i think im not explainig myself well, it's probably because of my english and im also finding this difficult to explain.

So basically this is what i need to do.
1. computer dials extension number from mexico to the us.
2. phone rings in us and it links a phone line in the us with the phone line from mexico
3. i now control the phone line in the us and i make a call to another computer.
4. and done..

Thank you i would really apreciate it if someone can help me out with this.

Yes, you can do this.
You need to work with your local phone company to get the line to the US.
At the US office you could have a computer that you dial into, that has a DSL or other internet connection. That computer would probably be able to use ICS (Internet Connection Sharing) to let you use the internet through it.
Your best bet is not to try to do all this on your own. After all, you need to concentrate on your actual business, not on learning how to set this up. So hire someone on the US end to set up that end.