🚀 go-pugleaf

I'm building a project which is essentially a microprocessor based phone 
answering machine.  I have completed all of the basic hardware and firmware
and am currently using a device that is manufactured by Cermetek (CH1813)
which provides a DAA for my device.  It also provides ring-detect, off-hook
detect, audio coupling to the phone line and a means for forcing the line
off-hook and on-hook using a TTL level signal.  However, this thing is 
expensive and there are number of other things that I would like to do.
I don't really have the experience to know how to proceed.  Any tips
on these things would be appreciated.  Also, any tips that indicate the 
reasons why any of these things could not be done, or would be expensive to 
do would be very useful.

Are there any books available whose subject is exclusively telephone
electronics, and includes sample circuits etc.? 

1) First off, I would like to duplicate the functions that the Cermetek
   device provides at a lower cost (< $26.00)

2) Let's say that I have a telephone hooked up to my phone answering
   device that has a ringer that cannot be turned off.  How can I inhibit
   ringing on this phone while still being able to detect things like the
   phone going off-hook etc.. I would like to be able to control this
   with the microprocessor so ideally a good answer would include 
   a TTL level interface.   

3) I would like to be able to use the DTMF keypad of a phone that is 
   connected to my device without causing the phone switching equipment
   to take notice.  Is this possible?  Maybe there is a sequence of tones
   that can be dialed to disable further interpretation?

4) How can I detect that a calling party has hung up before the telco 
   finally disables the connection?  How about when the telco finally
   drops the connection?   

5) Let's say that I have two phone type devices hooked to my device.
   I would like to be able to discriminate which of the two devices has gone 
   off hook.

Ed Archibald
ed@sybase.com