The company I work for makes a product that you can call into and it will route the call to either a modem /fax or voice line. You have a local device that you hook to your phone line and  when you call the device in the field it will first see if the call is for the modem or the voice port. 

We use the device so our customers do not have to have a second line installed for our control systems.

Thanks 
Tom

----------
From:   Byron A Jeff[SMTP:byron@CC.GATECH.EDU]
Sent:   Friday, August 22, 1997 8:43 AM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        Re: Slightly Offtopic, but PIC related.

>
> Hi,

Hello.

>
>
>         In one of my PIC projects, I am looking at connecting a PIC system
>         via a modem to the standard phone line.

That sounds like fun.

>
>         My problem here is that my phone line is also used for voice.

As are most.

>
>         Is there a mechanism that can be built or that I could buy that
>         would detect that it was not a modem and disconnect my PIC project,
>         thus allowing me to talk to the person on the phone.
>
>         I know I could get two phone lines, but I am looking at attempting
>         to provide a system that could sit on the side and wait until
>         I dial into the PIC system by modem.
>
>         If there is such a product, would it work with phone lines in
>         Australia.

Robert,

A good path the pursue is Distinctive Ring. This service adds a second number
and a different ring for a small monthly fee. There are circuits that will
route calls based on which ring is coming though. But anyone with PIC
experience can put one together quite easily using an optoioslator, a relay
and a PIC. The PIC would monitor the optoisolator for ring voltage and
would monitor the frquency of the ring. When it detected the correct ring
for the modem based device, the relay is then activated which connects the
phone line to the modem. The modem will then pick up successive rings
and answer.

Now that I think about it, you can probably integrate this into your existing
project simply by turning off the auto answer feature of the the modem. The
modem will then present RING strings for each ring. Your PIC can simply then
monitor the frequency of the RING strings and tell the modem to pick up when
the correct RING comes through.

I have a couple of ZyXEL 1496B modems that has distinctive ring detection
built in. I use districtive RING as my fax line (via the modem). Quite useful.

Hope this helps,

BAJ