Not sure where best to post this, I though it might help others. Mods, put in another part of the forest if you see fit. If you are in a hurry, just read the bits in bold.
My wife and I both have Voda pay monthly phones, both with European roaming. Last week went thorugh eurotunnel, arrived and my wife's phone got the voda welcome message ping, mine didn;t. Both are Huwei, both on SIM only contracts, been with Voda for aeons. After a bit I realised I had no signal, and as I was navigationg by google maps I had to go back in time and use memory, paper maps and looking out the window to find our way to the A26 and south. At the next stop I put it on airplane mode and off again, nothing, then a full reboot, still nothing. Rudimentary check of settings, all looked fine. Used wife's phone for mapping. Got to a hotel that had wifi and allowed wifi calling, logged into my account (I could receive the log in text because of the wifi calling, even in France, but teh next place we stopped had wifi but no calling), checked everything was in order. Went through all the settings on the phone, all in order, another reboot or two still nothing. Aside from mapping the other thing that was troubling me was credit/debit card use - if I had a security text from the bank asking me to confirm a transaction I might have ended up with cards blocked. I can live without a working phone socially but we en dup depending on them functionally. Next morning called 191 using wifi, got to an adviser but by unhappy conincidence the hotel wifi went down in a thunderstorm after the first few words. (when I did get a signal 2 days later I got a voicemail from the service agent trying to call me back - impressed).
2 days later I had a bit more time and carefully went through all the phone settings again. All OK, 2/3/4g set to auto, data roaming allowed, network selection auto. Then I noticed my wife's phone was on F SFR, what if ????. So despite dire warnings in the phone interface I turned network selection to manual, picked F SFR from the list that showed up, bingo it worked. Then I turned it back to auto (to do that you have to turn mobile data off, network to auto, mobile data back on) and it worked perfectly for the rest of the holiday. Some of it in remore places but it happily switched to other networks as we travelled. Got back tp UK last night, it did its thing and found Vodafone instantly. I have no idea why it happened, but the fix might be of use to anyone else in the same position. The other thing I wll do is set my wife's mobile number as the backup for log on security, having your home landline is as much use as a chocolate teapot when you are away.