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offers free conferencing for its users. On the sip network, you dial 1 747 222 1234 where 1234 is a random number you choose. Everyone who calls that number at that time is connected together. There are dial in numbers from PSTN at various points around the US, and perhaps the world, but it seems simplest to just hook a sip adapter between a phone and the internet, or to use a "soft-phone" (free ones listed on their home page).
Harold
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