Charges depend on many options. Orange pricing on *new* pay as you go connections can be found here: http://shop.orange.co.uk/shop/sim/payg_sim

If it's confusing - just deal with it in country. Call orange or walk into a store.

Calling a UK toll-free number of a London number from the UK should be charged at the headline rate as shown in the various tarrifs above. So it shouldn't be more than ~20p / min. Calling the US is likely to be expensive, though Orange may well have international calling plans you can add on which make it reasonably priced. One thing to note: receiving calls should be free.