Hi, Javier

there is no tone for detecting off/on hook. In telephone systems you have to
measure the current in the line. You can do this simple by putting the
optocopler serially in the tel. line.

    -------------------------------------

tel. line                                to telephone

    -------------|--|>|--|---------------
                 |       |
                 |--|<|--|

                     ||
                     ||
                     \/
                    ____
                   E B C
                   |   |

for the optocoupler you can use:
H11AA1 or TLP620 or PC814 etc.
With this transistor in the optoc. you can do a simple AF amplifier for audio
and for detecting dialing number.

Hope this help

Valter

Javier wrote:

> Does anybody know, how to build a circuit to detect how much time my
> brothers  used the phone and which number they dialed. I was thinking to use
> DTMF with 8870 and a pic to calculate the time, but don4t know how to
> trigger the start of time calculation, is there any tone for that??? (start
> and stop of the conversation)
> Anything would help thanks
> Javier