SparkFun Forums 

Where electronics enthusiasts find answers.

For the discussion of Arduino related topics.
By garik111
I have a project for arduino:
1. Connect it to phone line.
2. call specific number once a day.
3. After phone is answer arduino has to pass specific number through phone line.
I have tried to find some reference, however with no success.
By n1ist
You are looking for a device called a DAA. They used to be very common, but seem to be rarer now that everything is going cellular. Cermetek still sells them. Ixys and SiLabs both have DAA chipsets that would also work.

Phone lines are quite nasty places electrically (similar to cars...) with spikes, surges, and high voltage both normally (ring is 90v 20Hz, off hook is normally -48VDC) and when there's a nearby storm.

How do you plan to send the number over the phone line? DTMF, clicks, voice, 110 baud FSK?

By lyndon
As mike said, the standard device for this application is a DAA. If you can find an old serial modem, that may also work. You ATDT the number then send a modem command to go into voice mode and you can use in-band signalling for the information.

You can build a DAA from a couple of transformers and discrete parts if you're ok with crappy sound quality. Back when I used them, you could get some DAA's for around $20 at qty 1 (XECOM comes to mind), but last time I looked, prices were hovering around $100.
By n1ist
It's not that bad... Cermetek (which looks like it has the Xecom line now...) has the XE0092 for $25. That part handles the bidirectional audio interface, hook switch, and ring detect in a safe and legal manner.