Hi,
I my company we are currently moving our phone system to a freepbx voip system 
the first step was to install one freepbx server (that i will name PBX_A) with a SIP trunk to one provider with 2 IP phones and some softphones
this was done without any issue
The assigned internal phone numbers are short two digit
we will install a freepbx server (PBX_B) on another location with its own SIP trunk (that is not an issue as it is the same as our first freepbx )
now the fun part :
we need to make both freepbx server communicate with each other
both location needs to have short internal numbers
but both location should be able to reach people without using the SIP trunk
for example the person with PBX_A number 10 wish to reach the person with PBX_B number 11
we defined a quick convention : number starting with 9 are for internal multisite call
then comes a 2 digit location code, then the internal number
so PBX_A has location code 01
and PBX_B has location code 02
so the person at PBX_A with internal number 10 should dial
90211 to reach person a PBX_B with internal number 11
one extra difficulty would be that long number should also work
for example if PBX_A user 10 by mistake typed 90111 it still would call the direct number 11
can someone guide me how to do this ?
Thank in advance