Small Telephone Exchange.

Small private telephone exchange, initially meant to make 2 modems (one connected to a "client", the other to a "server") communicate with each other to test a server/client system before e.g. putting the server on the web.
Warning: It is not the intention to connect this device to the public telephone network! It has 2 telephone connections and the system is completely symmetrical: both can be connected to the client or server modem. The exchange gives a dialtone (420 Hz here) to the "client" modem (the one that picks up first), and rings up the "server" modem.
Currently the exchange can not detect the actual DTMF tones from the client modem. In stead a waiting time of 3 seconds is established before ringing the server modem.
Of course the exchange can also be used with 2 "normal" telephone handsets.
Circuit diagram, State transition diagram