Thank you Audioguru and Keepitsimplestupid for the suggestions to research. I will check into that.
To Rexlan, I think you may have mistaken me for a noob hobbyist. I have already developed PIC code for generating extremely accurate xtal locked DTMF sine frequencies and can easily be twist compensated, see
Zero-error 1 second timing algorithm. Secondly, most cheap answering machines use a transistor to pull the line low and take it off hook, it's a very common system used in commercial products. Thirdly, there is nothing illegal about researching and developing a potential new phone product.
Please speak up if you have anything of value to add, your reply seems to indicate that you have some experience in this field and I would much appreciate any input you can provide.
Thanks to all.
(edit) OK, I've read up on the "call progress tones" it was mainly stuff I had already seen. This is still looking like a worst case scenario where the PIC will need to recognise the ringback tone and detect when it ends when the person picks up.
Audioguru, can you provide more info on the pulse you said was provided when the person picks up? We get a "click" sometimes here in Australia when they pick up but I thought it was more a switching artifact than a deliberate and reliable system to detect pick up?