ATM Three with unlimited everything for data calls and texts.
PAYG SIM for incoming, currently on O2 network GiffGaff, (but I could change that any time to another PAYG one, whichever seems best). This is my regular number that is given out to businesses etc.
Friends and family know both numbers, if one doesn't work they try the other one. Or send a text. Or leave an answerphone message.
These both live in a twin SIM 'phone, to which I tether.
By the nature of my 'phone (dual standby, not dual active), only one SIM may be active on 3G/4G (the Three one) whilst the other can only use 2G. So the PAYG network for incoming voice is chosen for best overall 2G coverage.
When abroad, Three data fair usage is 20 Gbytes/month in the EU. I find that sufficient since I don't stream a lot of video. If I did run out I could pay GiffGaff for a data package for a month and get, I think, up to another 20 Gbytes fair usage, but that would knock out the Three SIM because it doesn't work on 2G. Or I could get a data boost from Three instead and carry on as normal.
Never had to do that yet.
If I wanted a more serious setup, such as a
MiFi, maybe with an external antenna, I'd pop in the Three SIM and carry on, business as usual on the 'phone. If the signal is weak (mine is a panel van, all steel) I have occasionally put the 'phone into a waterproof clip top food box and put it on the roof, reaching through a skylight. Can't hear voice calls that way, but there is always the answerphone and missed calls log, and must remember to bring it in before setting off, though I do have a tether fed back through the skylight so it wouldn't be lost.
Works for me, I am not an "always on" person, obsessed with being always connected and in touch.
Basically a twin SIM 'phone is my recommendation, mine is a Moto 6, Amazon themselves supply their special version, with extra memory as well, otherwise twin SIM 'phones are hard to come by in the UK. but very popular elsewhere.
Apple people can't do this, AFAIK.