Making PSTN phone calls using Asterisk

I would like to develop/use an application to make phone calls freely around the world from PC. Or I would like to develope an application from which I can connect two phone numbers to talk freely around the world from the computer connected to Internet.
I have used an application from BT SDK to connect two phones anywhere in the world from the computer, but now they removed this after merging with Ribbit.

Can we Asterisk for connecting two phones any where in the world? As we use Internet and VOIP and can we make this freely available for everybody?
Please help how to develop such application using Asterisk open source code.

Thank you in advanceā€¦

Asterisk can do it. The limit to the amount calls you can have at once is based on the server that you have. Have a look at the book: Asterisk, The future of telephony:

also poke around