on 28-07-2019 14:00
on 28-07-2019 14:36
This isn't customer services so to contact O2 see here http://www.o2.co.uk/contactus
However, you might want to look at the international bolt on https://www.o2.co.uk/shop/international/the-international-bolt-on
on 28-07-2019 16:16
All your phone's will work in the USA already but you will be charged £4.99 per day on the days you use them.
The international bolt on is no use to you at all as that only applies to use in the UK.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)