Your first step should be to contact your PBX vendor, SYStech.
Unfortunately, I can't seem to find a PBX manufacturer named SYStech on the web. Can you tell us the exact model of your PBX?
If SYStech is not helpful, then you will have to go looking at radio-telephone interconnect, such as those manufactured by
Midian Electronics.
GAI-TRONICS, or
Barrett Communications.
Do you already own the two-way radio system you wish to connect to your PBX? If so, what is the manufacturer and model?