Hi, if the fourth (far most right hand) button is marked 'Trans Rlse' this particular model of phone and its associated extension was intended for use only with a PAX and was produced by Ericsson/Plessey and distributed by Telephone Rentals and (possibly) other private rental firms.
I had a brochure somewhere with details of a number of these systems, unfortunately I have mislaid it
Regards
Andrew