I have an old Executone Encore KSU816 Model 2513501. I have 14 speakerphones with 12 buttons. The speakerphones all have microphones in them but I can never be heard when "on hook". I can page but have no intercom function. I have no manuals of any type. There is a vacant card slot labeled "OPT". Could this me needed for speakerphone operation? Could this be needed for the intercom function?

Avcom1
The 2512501 is a 6 button standard phone. The 2512502 is the 6 button speakerphone. On you model the 501's I believe you can make a reply through the intercom hands free. (Your handset down with another extension calling you)