3 FreePBXs with the same numbering?

Question

Asterisk

Good day colleagues. Perhaps the question is stupid, but I can not understand how to solve the following problem.
There are 3 offices, all of them have the same internal numbering. You need to configure intercompany between three offices in such a way that, for example, by pressing button 9 in office 1, you can call office 2 numbers. By pressing button 8 in office 1, you can call office 3 numbers.
Or some other solution.

Answer 1

Here it is necessary to smoke the documentation on outgoing routing , especially in terms of Dial Patterns.

Answer 2

It will be easier to add one more digit to the numbers for each office, with the office number. in general, make a new numbering.

Answer 3

roswell solalex
Comrades, did everything on the VM, made 3-digit numbers, raised the SIP trunk between Asterisk. When I call through a prefix to another aster, this is the problem
[2018-02-06 14:55:59] WARNING[7754][C-00000019]: chan_sip.c:17293 check_auth: username mismatch, have <101>, digest has
[2018-02-06 14:55:59] NOTICE[7754][C-00000019]: chan_sip.c:26361 handle_request_invite: Failed to authenticate device "Director"