I am totally aware of this issue Andy. However....the scheme that is effective is to define the number of rings the gadget looks for at first to be, State 0: Looking for Rings, "more than M" and then after the time when the normal ring detection cadence ceases (ie. the remote site has hung up) then enter the case of State 1: I got the M or More Rings. Then you enter a State 2: Idle Time that times a nominal time period of say 5 to 15 seconds. If another ring cycle happens during the valid window of State 2 then you enter State 3: Looking for Second Rings and once again look for "more than N" rings. Once the more than N rings state is satisfied the gadget seizes the line. From a user perception at the remote phone she first lets the phone ring back in her ear M+2 times (or more if so desired) and then she hangs up. Now she must dial again within the nominal window so she tries to shoot for 5 to 10 seconds or so....not too difficult. She then lets it ring in her ear a minimum of N+2 rings or until the gadget picks up the call. At this final moment it is still a good idea for the gadget to emit a characteristic beep that says "I'm here" and then wait for the remote user to press in a 3 or 4 digit DTMF code. If the correct code is recognized then the unit will emit another characteristic beep that indicates "I know you". If more robust security is desired have the gadget keep polling for the DTMF digits and only accept the last 3 or 4 entered and do the validation only on those. The gadget detects "end of DTMF" input by a timeout of a normal DTMF interdigit time. This method goes a long way to foil would-be hackers.

Now back to the ring counting. The method above works when the prescribed repetition rate of the ring back tone in the callers ear matches the ring cadence rate at the gadget on the phone line. Now there may be systems in some places where this is not the case but I have always had this scheme work for me. One product used this scheme back in the '84 time frame on a 8051 processor.

Michael.....

Can I have back the fur you stole from the end of my tail?