8 Tone generation
Every decent telephone system needs to give feedback to it's user as to it's current operational status. This ha s been done since time immemorial by using progress tones. I only need three types of progress tones; a steady tone for showing the exchange is ready for a call, a ringing cadence progress tone, usually two short bursts of tone separated by a second, and a "fast busy" cycling tone to show a false number has been dialed. So what we will need is some way to generate these tones and have them fed to the user operating the phone. I'm sure there are very many ways to generate a simple sine wave but I have a job lot of NE555 timer IC's so that's where I'm going to start. Using a NE555 timing calculator (this one is my favorite as you can enter the desired frequency and it will generate the options for the resister/capacitor combination to achieve it). I'm not too concerned about the cycle lengths since I'm going to feed both outputs ...