[asterisk-users] One server, multiple companies

[Eric C.]

Hello all, 

Just starting to setup asterisk v 1.4.11 and need to run three distinct phone systems for three different companies.
So far, I have inbound lines going to the appropriate dial plan within the extensions.conf file. I'm using  

exten => _X.,1,NoOp(FROM NUMBER:  ${SIP_HEADER(TO):5:10})

to determine which number is being dialed by the caller and then using a gotoif to get to correct greeting (correct company).

My question is... lets assume all three companies have extension numbers being 2000, 2001 & 2002, how does one separate them?
Or, lets say the extensions are:

company A --> 2000, 2001,2002
company B --> 3000, 3001, 3002
company C --> 4000, 4001, 4002

Since they're on one server with one asterisk process, how can I use context correctly so that the user at 4002 cannot get through to the user at company A whose extension is 2000 as currently, I can dial 2000 from phone 4002.

That's my current problem, how should this be setup?  Is my architecture correct? Should I be running different processes for each company? Can context resolve what I need?

Please advise.

thanks,
Otto