net2phone/telephony for Linux?

I'm looking for something like a www.net2phone.com type of service that I
can use from our favorite platform. Does anyone know of something similar
that I can use?

Thanks

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Lucas Vogel, Software Developer
LandWarrior Project
Exponent Inc.
lvogel@exponent.com
phone:    (623)587-6739
fax:        (623)587-4187
------------www.exponent.com---------------