HDLC via V.24 or b-channel?

Hello everyone,

I'm not sure if this is the right audience for my problem, but I think 
you guys know how to speak HDLC at least :)

I want to program an application for a PBX. The PBX is designed to be 
connected to a PC and offers 2 interfaces for data exchange.

Unfortunately, I don't have a full description of the protocol(s) used.
I do have information about the protocol for implementing applications, 
like "How does the PBX tell me that extension 12 is ringing" and so on.

I don't have information about the lower layers / protocols, besides 
these facts:
The PC connects to the PBX via
- V.24, 9600 baud, asynchronous
or
- B-Channel
Additionally, connection schemes of installation instructions do note 
"HDLC" between PC and the (V.24-)connection to the PBX.

There has been a whole software/hardware-package provided by the 
manufacturer using a propietary interface-card called. As far as the 
installation manual for the original software reads, the system was 
designed using a SCO UNIX and a "HDLC Primary Device Driver".
The interface card hold two modules, called "V.24-module".

What I am not sure about is the question, how HDLC might have been 
implemented via "V.24"? Would it be possible to do this via a standard 
(USB-)RS232-interface?


Thanks in advance,

Jens