How to connect another Asterisk servers and transfer calls

Hello. It’s my first time on this forum. I have a question. Ex. I have Asterisk server and some SIP users. They have external PSTN numbers from 1-10. And example another company have Asterisk server and some SIP users also with external PSTN numbers from 20-30. And now If user from my network want to call to number 20 will be connect via my VOIP provider and I must pay for that. My question is how to do that ex. when my user will be call to number 20 the connection will be goes between my Asterisk server to Asterisk server from that company where he call excluding my VOIP provider (connection between servers via Internet will be free) And the same work when user from another company with number 20 will be call to user with number 10 in my network. It’s a good idea to start a project like that to connect together Asterisk servers. Please for help with my question. Thank’s. Bye