Different carriers can use slightly different 'standard' sim sizes (the important metallic part of the sim card). My experience was that the T-mobile (UK) sim was too large to cut without damaging the actual sim. So I had to nip down to the local T-mobile shop and they were kind enough to sell me a duplicate micro-SIM for £10GBP. In most cases the cutter option should work fine, I'm just saying be careful and take your time. :-)