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:

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.