Arduino Whole House Telephone Intercom
Using the Intercom - "Flash" to Operate
Intercom Mode: "Flash" a premises telephone off hook, i.e., take it off hook briefly, then hang up again. (I define a "flash" as a duration of 200 mS to 2 seconds.) The intercom temporarily disconnects premises telephones from the Telco line, and rings them with a distinctive cadence. After a suitable ringing period, pick up your phone and wait for someone to answer. When you take your phone off hook, (or someone else on premises goes off hook,) the intercom stops ringing and supplies premises phones with talk current. All off-hook parties can converse. When all premises phones go back on hook, the intercom re-connects them to the Telco line.
If an outside call comes in while talking on the intercom, you will hear the ringing signal in your earpiece. If desired, you can hang up all premises phones to let the call ring through, and then answer normally.
Intercom/Hold Mode: This is useful when you answer an outside call that is for someone else. Flash your telephone on hook. The outside call is put on hold, and the intercom becomes active. Hang up to activate the intercom's distinctive ringing cadence. Pick up your phone again to talk with another person on the premises, all while the caller is on hold. Tell the other person to flash on hook to receive the outside call, and then hang up. Flashing on hook again ends Intercom/Hold mode, and reconnects the outside caller.
Note that an on hook flash only works if just one phone is off hook. If all phones are placed on hook in Intercom/Hold mode, Intercom/Hold mode remains active and the distinctive ringing cadence is heard. To exit Intercom/Hold mode, you must perform an on hook flash.
Normal (Transparent) Operation: Most of the time, the intercom transparently connects premises wiring directly to the Telco line. Place and receive calls as if the intercom were not installed.
Flashing the Telephone Company: If you want to use a phone company feature that is activated by an on hook flash, such as responding to call waiting or three-way calling, you must double flash instead. That is because the intercom responds to a single on hook flash by entering Intercom/Hold mode, and does not present the flash to the external phone line. The second flash (if begun within 2 seconds after the first flash ends) is forwarded to the external line, and cancels Intercom/Hold mode. (Alternatively, you can disable Intercom/Hold mode for your installation.)
Ringing Timeout: If the intercom is left to ring premises phones continuously for 2 minutes, ringing timeout occurs. The intercom reverts to Normal (transparent) mode. If the extended ringing period was in Intercom/Hold mode, the outside call is dropped.