How to create attribute combos?

[cyberdoc]

Hi guys, am new here and excuse me if this is too basic. Basically what i want is 2 combinations for a product. ill explain with an example. Ive a mobile hardware store. So it has a product(Ex. iPhone 6), with an attribute country (US, UK etc) to select which country the user belongs and depending on the country there are operators to be available(2nd attribute- Vodafone for US, ATT & Verizon for UK, Airtel & Idea for India). How do i make sure, the user sees only the respective operators when he selects a particular country from the attribute dropdown...

Waiting for reply, 

Thanks in advance