That is a extrange PBX. Are you sure that is not PRI???
If it is BRI (could be, but never seen one),you could put a ISDN modem to convert from BRI to analog port to connect to Quintum A200 and A400. Not too nice looking but it should work.Thinking it over , that will be hell to get disconect signaling to work. You better stay away from it,
At least on the U.S. used to be some BRI to PRI T1 converter. They were used in places where the C.O. was unable to give you digital T1 spans , but they could provide BRI.
I have not seen one of those boxes in years, but maybe E-bay will have one. Try google search.
Still the best will be get a T1/E1 or normal trunk into the PBX if posible.