phone wise, i have the unlocked Samsung S10+ Its a dual sim phone (If bought in europe, Not the US version), normally at home I use a Vodafone sim and a micro SD card, but if i'm abroad in a country that isn't on Vodafone's cheap/free roaming/data i just remove my SD memory card and replace it with the local network sim, (The phone has 128gb of internal memory so i can easily do without the memory card). the two network sims work perfectly.
Its easy enough to switch between the two sims (uk one and the one from the country i'm in). it allows any emergency calls etc from the UK to still get through on your usual number that most of your friends and family know, whilst also giving you the cheaper tariffs from the country your in for local calls and data usage, whatsapp, messenger, Skype etc etc only require access to the internet to work perfectly, it doesn't matter if that's your home internet, some hotel WiFi or a local provider data. I find Whatsapp is ideal for free calls home, not that it matters much but it is that secure even the UK military are conducting a lot of daily business using it.
Vodafone UK will charge me £6 a day (even If I only sent one text/read an email online etc, that would automatically trigger the £6 fee for that day ) to use my UK tariffs (Unlimited) while in the States, if only there for 2 days its probably not worth getting a US sim, but any length of time or constant visits would soon build up the £6 per day rate
3 do a sim in the UK for 1 month at £25/26 that allows you unlimited Data/Calls/Texts while in the US, swap your phone sim or if you have a dual sim phone even better. Im not sure how good 3 are or whose network they bump onto over there, but i hope its better than their efforts in Scotland
the phone itself has a great screen size for viewranger/mapping etc, is waterproof and has a very good camera, battery life depends on usage, all day on the net i charge it twice a day (its actually my computer these days, i don't use a tablet or laptop anymore so it gets very heavy daily usage, probably about 9 hours online daily), when hiking in airplane mode with viewranger tracking it uses maybe about 30% a day, maybe less.
my last Samsung the s9, also had dual sim, not sure about the versions before that or other manufactures but there is going to be cheaper 2nd hand phones about with the same capability (Battery life on my S9 was really bad though, wifes was ok, the S10+ is far better) Pretty much most UK smart phones will work on the US networks, cost of usage is the bigger issue.
NB - this thread piqued my own interest as i'm heading there in August for 3 or 4 weeks, so i just spoke to Vodafone and it seems i'm already on their unlimited max plan so i get to take my uk unlimited plan without having to pay the £6 daily charge, Wish i had actually asked them in the first place, it means i can stop looking for a best deal US sim card myself so many thanks.