I am building a device that decodes DTMF tones over a phone line to
turn on and off devices at home through relays. I use a PIC
microcontroller in my project. My problem is the interfacing circuit
with the telephone line. I have already succeeded in making a ring
detect circuit that will detect when the phone is ringing. Now I am
wondering how the microcontroller would answer the phone. What kind of
circuit would I need for this? Something suitable to Canadian/American
telephone systems would be appreciated...

It may be illegal to connect home-made device to public network.

Instead, use standard auto-answer modem to pick up the phone and
interface I/O port of the computer to your device.
http://www.modemsite.com/56k/autoanswer.asp

Also, look at X10 devices
http://www.oreillynet.com/pub/a/network/2005/01/10/x10_hmhck.html

and

http://www.blackbox.com/Tech_Support/Technical-documents/KVM-switch-over-IP.aspx

http://www.networkworld.com/reviews/2001/0820rev.html
http://www.c-sharpcorner.com/UploadFile/mgold/RemoteSensing08292005100245AM/RemoteSensing.aspx?ArticleID=f3bd03fe-a9fd-4f81-b50a-57459a0968da

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Request for Answer Clarification by shaun_ca-ga on 02 Aug 2006 00:32 PDT

This is a design project and I've already committed to use it on a
phone line. Even if not using it on the phone line directly, I might
hook it up to a EPABX. But I do need that circuit. Do you have
knowledge of the circuit?

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Clarification of Answer by hedgie-ga on 03 Aug 2006 11:25 PDT

EPABX was not meantioned in the original question and
 I never heard of it until now in your clarification request.

 Anyway,  a search  and got:


PABX (Private Automatic Branch eXchange): is a telephone exchange
operated within an organisation, used for switching calls between
internal lines and between internal and PSTN lines. A PABX can route
calls without manual intervention, based entirely on the number
dialed.
  
EPABX (Electronics Private Automatic Branch eXchange): EPABX is a PABX
system which is built using electronic controlling and signal
switching (to differentiate them from some old relay based designs)

http://www.crosby.co.uk/technical_support/guru/howitworks_pbxsystems.php

This request may be of interst to you:

(start of quote):

" I want to know the principle and operation of an EPABX, please ..

PABX (Private Automatic Branch eXchange) is a telephone exchange
operated within an organisation, used for switching calls between
internal lines and between internal and PSTN lines. A PABX can route
calls without manual intervention,


This is just basic quicly written down simplified description how
those things work...

: Can and EPABX be desinged using Microcontroller 89c52. 

Maybe if you don't need very many lines or most fancy features....

: If yes, kinldy suggest some idea for the same.

Here are some links to some PBX and telephone information which coudl be useful:

 " (end of quote).


http://www.epanorama.net/wwwboard/messages/6124.html


It looks fairly complex.

Here is a technical manual (availablle for $50)

Siemens Saturn T1 EPABX Interface Manual

$50.00


Availability: in stock 

http://www.twacomm.com/catalog/model_1550013.htm?sid=18C9E6D3E77B1D4F2AAD6F048384624F


If you need more info on  EPABX , rather then info on 'how to
interface a single line to a device', I sugest you post another
question and describe desired info.

Here are few more links. It looks like  EPABX heavily used in India today.

http://www.kellysearch.com/qz-product-3931.html


Hedgie

I asked specific information about an electrical circuit. However, in
the response I was told to use a modem to interface my circuit. This
is not what I want and something I could have figured out on my own if
I were to go that way. A request for clarification has not yet been
answered.

A short across the line should answer the phone.

A huge waste of time searching Google for

microcontroller telephone module -gsm -wireless

will likely to produce a result withing a few days of searching. This
is why I like to leave researching to GA and other professionals.
However, I did pick up this:

http://www.circuitcellar.com/library/print/1004/Zhang/index.htm

I would search within companies like Parallax, Freescale
Semiconductor, Rabbit Semiconductor, magazines like Nuts & Volts,
Circuit Cellar, etc. Funny that Ethernet modules are now made by about
every such company. Good luck.

I would suggest you to use some ready made ICs from Analog Devices,
BurrBrown(now Texas Intruments), ...
use a search engine like http://www.globalspec.com/ to get to know the
manufacturer of those kind of ICs

Infact using IC can simplfy your design. Drastically reduce errors and
chances of failure.
Other keywords you can use to search are USART,  modem IC etc..,  
Good luck in making your circuit

I'd like to thank everyone. The search is progressing and giving good
results. Everyone except the real researcher to the question has been
helpful.