I joined the FON network, and use a combination of Skype and wireless access via my phone to make calls. It works well for most places I go to.
With FON, you install an open access point in your house, and then you can use other peoples access points for free. It has great coverage in some cities, but not all of them, but if you don't need to be able to make calls absolutely everywhere, its a cheaper option than having to manage tons of sims.
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