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VoIP Analog Phone Adaptor

A $20 project that mates with your sound card and lets you use any old phone for voice over internet to save a fortune on phone calls

By Ross Tester

Click for larger image
As circuits go, you cannot get much simpler: one transformer, one resistor and one battery, plus connectors!

In fact, we featured such a phone, from Microgram Computers, in the "new products" section in August 2004 (followed by a full article on VoIP and Skype in September ’04). Those phones are still available and just as viable as they ever were. But they aren’t all that cheap. And using a headset/microphone, while once again perfectly viable, can be both a help and a hindrance. It might free your hands but it just ain’t the same as holding a phone to your ear.

You can also buy "internet phones" without any problem these days. While initially more expensive than standard (POTS) phones, they offer significant – and often dramatic – call cost savings in the longer term.

But what if you wanted to get into VoIP with a minimum of upfront cost? Wouldn’t it be nice if you could use just about any old (or new!) phone on your PC instead of buying something special. After all, most hobbyists and even significant numbers of the general populace must have a phone they are no longer using, gathering dust somewhere! Even a brand new phone can be found for less than ten dollars!

Don’t look now . . . but you can! All it takes is a tiny bit of cheap circuitry to fool the phone into believing that it is plugged into a normal call – and it can then connect via the sound card in your computer to the internet and the rest of the world!

We’re not going to make any claim to the originality of this circuit. We were alerted to a website containing just such a project by a reader and it seemed to us that this would make a dandy little item for SILICON CHIP. You can see the original at All the information presented there is given here – with the exception of a rather long user forum which itself might answer some of the questions you have about this project.

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