where to send packet from a PC

i want to write a software in c for dialing to a PSTN phone.
i dont have any gate keeper or MGCP. tell me how to code everything in the software itself that it will perform all necessary task. without putting any hardware device.
setup will be
Phone—-PC—Internet—-PSTN—-phone.

the second end phone can be anywhere in the world.

so can u tell me the algorithm to code it?