A commercial Electronic House Arrest Monitoring system I designed uses this ring counting/timing method quite successfully for those odd occasions when the monitoring center wants to contact the field unit to perform some on-demand test. Normally the field unit originates the calls. The "client" is instructed not to answer the phone until the third ring and the client has the incentive to abide by this request, since interfering with the system may qualify them for the alternative form of sentencing; that is, jail. Both ends are designed to deal with lack of synchronization between the called party's ringing signal and the caller's ringback, and use a ring-once (or thereabouts) to "arm" the field unit into an auto-answer mode, hang up (pause), then call back procedure. The client, hearing one or two rings, knows that this call and the next one are destined for the monitoring device, so does not answer the phone. Works like a champ! Oh, and both ends use state machines to implement this particular feature :-)