I have two asterisk servers on two different networks, my first one delivers numbers from 100 to 105 and my second one from 200 to 205. How is it possible to call the 201 number from the 101 number phone?
If yes, what is the configuration file to be modified and with what variables?
You almost certainly also meant to say using SIP, as this can also be done using analogue, PRI, ISDN, H.323, etc.
You define an endpoint for the other system, in pjsip.conf. Ideally do so by IP address, not registration (one has to be by address, in any case,) and, optionally add authentication. You handle outgoing calls (acting a SIP client), to the other instance, as you would an outbound call to an ITSP, and handle incoming calls (acting as a SIP server), as you would from a phone.
Noting that server refers to the hardware, and not to Asterisk, and that there isn’t a need for the originating user agent to have an extension number (unique or otherwise), it is very easy if you have a basic understanding of how Asterisk configurations work.
A peer support forum like this is best used as a source of hints, or to help debug specific problems. It is not a substitute for courses, or paid services.
You could also consider using FreePBX, in which case the construct you want is an intra-company trunk.