I just wanted everyone to know that I finally did get my system up and running. For the record, It CAN be wired for a Modular connection. I used a simple wall Jack to complete it, but I had to cross the lead where the transmitter is waiting for the Telephone to send the connection that it's on the hook with the same lead that comes of the R2 Connector (Yellow or Also known as A1). This way it thinks the phone is always on the hook. It won't do On the hook dialing but everything else works fine. You can turn it on and leave it on while you dial on the phone without any feedback. When you hang up the phone it will resume connection.

Also, I did this with a type 4B speakerphone which is the precision components version, however, after comparing the schematics for both systems, it's the same procedure for the 4A. The only difference is the color of the lead cables in each set is different. If anyone needs help how to hook this up to single line without a 2500S or 500SM single line phone let me know.