Asterisk, same extensions but different companies. How?

Question

Asterisk

The point is this.
There is one instance of Asterisk. There are 4 companies (4 different numbers).
How to make it possible to enter the same internal numbers for each number (company)?
for example, there are numbers XXXXXX and YYYYYYY, and each has an internal (extension) number 100.
A user with XXXXXX connects with a softphone to sip1.site.ru with a login of 100 and his password
A user with YYYYYYY connects with a softphone to sip2.site.ru with a login of 100 and with your password

Answer 1

What will the phrase "different contexts" tell you?

Answer 2

Why does everyone who asks about Asterisk want to serve exactly four different companies with one server? Is this some kind of flash mob?
Let me guess - all your incoming numbers are mobile, from GSM networks ...
Theoretically, one Asterisk has the ability to serve an unlimited number of companies, moreover, they do not suspect anything about each other. If only there was enough power.
You cheerfully describe to Asterisk the entire periphery of all companies and start weaving tricky dialplans, i.e. routing programs for the incoming connection request. They are usually written in a specialized scripting language.
A separate structural part of the dialplan is the context. It may be one, or it is possible that the dialplan consists of several contexts connected according to some logic.
Each peripheral device (terminal) has a context, in the cat. he can send his connection request. And nowhere else.
Unless you mess up context nesting hierarchies, the call will never see those it's not supposed to know about. And it will go through the routing chain of a specific call, which describes exactly his company. Or a division.