Skype works fine on Linux. And calling a US landline over the internet is quite cheap, I use Skype to call Canada from abroad (for reference, from Guyana, Barbados, France, etc...) and it costs me around $0.02 Cdn per minute... Check out their rates: http://beta.skype.com/en/rates/#learnMore
Not that I'm trying to sell you on them, but I've found them to be very reliable, the Linux and Android apps work fine, and it's convenient.
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