help with iphone 5 please

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hi If i have a iphone5 which is on contract with Vodafone and i get it unlock will my phone be affected in anyway? Basically i have the iphone 5 on contract and I will be moving overseas for 3 months due to work, so i would like to unlock the phone so i can use an international provider as it would be cheaper cause im going away for a long time. Also if i get my phone unlock will i still have to pay my monthly bill at Vodafone? Im just trying to find the cheapest solution for Iphone 5 any help is greatly appreciated. On a side note if i wish to send a iphone5 as a christmas gift to my boyfriend who lives overseas due to work can he unlock it and use it overseas and i not be charge with monthly bills?