You will be using your mobile internet on your phone then. First and foremost this will be what you pay for (Skype audio and video calls + IM). If you call someone on their phone you have to pay the "pay as you go" rate ( http://www.skype.com/intl/en/prices/payg-rates ) or have it included in Skype Premium or a Skype subscription.
I recommend to download the latest Skype and search first.
To get the most out of Skype I recommend Skype Premium.
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