I saw an interesting page about this subject:

http://www4.tpg.com.au/users/talking/TaxiPhonePage1.html



On Thu, 13 Dec 2001, Tim McDonough wrote:

> Has anyone been involved with circuits to take a phone line off
> hook and then dial a number on command from the micro? In
> general, the parts I'm still researching are:
>
> - Detect if the phone line is already in use without disturbing
> an in-progress call
> - Go off-hook and dial a number (suggestions for the tone
> generation part?)
>
> The device/phone line connection should be a pre-approved DAA. We
> don't want to jump through all those hoops ourselves unless
> there's a big cost savings at the end.
>
> Low cost is very important. I'm trying to avoid having to
> purchase any of the available OEM modem modules or the stand
> alone dialers that I've found so far.
>
> A modem built up from the Silicon Labs Si2400 chipset runs around
> $21-$27. The Cermetek modem modules are around $35. We don't
> really need a true modem for the information we need to send. It
> seems like it should be possible to do a much simpler dialer for
> half to two-thirds of those costs.
>
> Thanks for any reasonable suggestions,
>
> Tim
>
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