if i were to buy a mobile phone from the usa, can i use it over here and do i get a sim card from any mobile phone shop? also is this the case vice versa in the usa with my uk mobile phone that i have now? i dont want a contract, just top up.
Most cellphones don't have sim cards at all. The sim is hardwired directly in to the circuit board.
We visit there a lot and have have cellphones which are pay as you go, but can't use them here obviously.
You can use your mobile in america, as long as your free roaming is activated... but it will be cheaper to buy a pay as you go cheap cellphone than pay the ridiculous per minute charge.
Most of the Americas, including US & Canada, use GSM frequencies of 850MHz and 1900Mhz for mobile phone communication, whereas most of the rest of the world, including Europe & UK, use 900MHz and 1800MHz.
Thus for a mobile phone to be compatible for use in both the US and UK it must be able to operate at these frequencies and will be described as multi-band, tri-band, or quad-band, depending on how many of these frequencies it can manage.
Not sure what your circumstances are but this is what I do ... I have two phones, one in USA and one in UK and I top them both up. That way I don't incur all sorts of charges using it in US etc. It works out cheaper in the long run and when I am in US I just switch off the UK one and vice versa.