I have another angle to see this issue (might be wrong, please correct me).

Because the ring width and frequency could be differenct among different phone switches. Would it be better off to use time interval instead of ring count? Say, let it ring 20 seconds instead of 4 rings. By doing so, you get three benefits.

1. It is more intuitive for the end-user to set the time of waiting instead of rings of waiting. My office phone beep twice for every ring. Does it mean one ring or two? Confuse me as well as your program.
2. No additional hardware required.
3. No software tweaking is need. You start the timer on the first ring interrupt. On every following interrupt, you check the timer expiration and do the rest actions.

Regards, Oliver