This interesting circuit found in American documentation generates a busy signal, without you having to take the phone off the hook if you do not want to answer. The components used are common.
Found in American documentation from the 90s, this circuit has an input resistance of 100...
Found in a 1967 RCA manual, this circuit uses components from the era that can be replaced by modern...
Suggested by Philips in a 1970 publication, this circuit uses components from that time. No...