Interconnecting FreePBX and ERICSSON BP250

I want to insert a FreePBX box between the legacy ERICSSON BP250 PABX (currently connected to the PSTN) and the PSTN itself to provide new functionalities to both current extensions, connected to the BP250, and the new extensions that will be connected to the FreePBX box.

The FreePBX box shall have two Primary Access lines interface, one line going to the PSTN, the second to the BP250 like this:

       PSTN <------> FreePBX <------> BP250

The whole has to work transparently for both new as well as current extension users and external callers.

I’ll appreciate your comments on the various aspect of this exercise.

(1) Ext-to-ext calls (within each box)
I assume that if I assign a different first digit to the extensions under the FrePBX wings from those of the BP250, each PBX will handle the call without any trouble.

(2) Ext-to-ext calls (from one box to the other)
On the FreePBX, I shall create the trunk linking with the BP250 and an outbound route that selects it when the first digit indicates an BP250 extension.
Do you agree?

(3) Inbound calls
When an external call arrives, no matter how many handlers will go through, in the end the call has to go to either an extension or be forwarded to a trunk line.
Should the target be on the BP250, should I create a custom destination to handle the call or…? How should I configure it?

(4) Outbound calls
When a FreePBX extension generates an outbound call, outside lines trunk(s) + an outbound route and that’s it.
When instead the caller is a BP250 extension, then it will come from the trunk line.
How do we discriminate if it is internal vs. external and how do we process it in the two cases?

(5) Handling of features
May we transparently provide the Asterisk various features also to the BP250 extensions? How?

May be this is OK for a start…