Since the UK is (at the time of writing) part of the European Union, I've been spoilt for the past two years by "roam like at home", whereby I can use my UK mobile phone in other EU countries for no extra cost.
However, a nostalgic reminder of the good old days before 2017 arrived when I landed in San Francisco:
Yasss, you've arrived in USA ? it's a Go Roam Around the World destination! If you're on an Advanced Plan you can use your monthly call and text allowance (Fair Use limit applies) and up to 12GB of your data allowance at no extra cost. If you're on a 12GB plan or above, data usage will be capped at 12GB per month. If you're on an Essential Plan it's £1.40/min to call, 99p/min to receive a call, 35p/text & £3/MB. Go to three.co.uk/go-roam for full terms. Safe travels.
Clearly, it must be impossible for Three to look up what plan I'm on and send me a message which matches, i.e. one saying "£1.40/min per call / 99p/min to receive, 35p/text & £3/MB - better hope nobody calls you, and be sure to take out a second mortgage if you want data!"
Most of this shameless gouging is easily avoided by buying a local SIM card, but what about making voice calls back to the UK? I bought a data-only SIM, partly because the call prices didn't look especially encouraging.
For friends and family under 40, calling via Facebook Messenger/WhatsApp/Signal works well (I'd always prefer Signal, given the choice). But of course there are certain relatives who only have an actual phone.
As it turns out, the classic solution is the best: use a UK VoIP account, e.g. from Andrews & Arnold for about £1.20 per month plus the (single digit pence per minute) call costs. This hooks up to a variety of clients on Android e.g. Linphone, and means I can make voice calls anywhere I have sufficient WiFi or mobile data bandwidth to support one (which is not very much - it even worked in Yosemite National Park with a sliver of LTE coverage).
The only glitch I encountered was one-way audio, a bit of a classic when making VoIP calls over NAT. The solution turned out to be simple: force the use of IPv6. Linphone supports it, my US SIM supports it, and A&A support it. VoIP is the killer application which functions much better with proper end to end IP connections and no NAT in the middle.
I can only apologise to the staff and shareholders of Three - no doubt some people will be funding your next holiday with their roaming charges, but I won't be.