I bought an iPhone 6 from Apple in the UK and then got a monthly tariff with O2 in the UK. I am going to be in the US and would like to use my phone there without incurring huge roaming charges. Can I simply buy a sim card in the US and insert it into my phone and use it in the US and then replace my O2 sim card when I return to the UK? Are there any phone services in the US (Florida) that are better for coverage or quality or are they pretty much the same? Thanks for any advice.