I was one of those with this problem back in November last year and can confirm that R101's fix does work and I've had no problems since.
For anyone new coming on board with this issue, I suggest that it may be worth reading through the various comments from about page 6 onwards as there are some other little twiddles that you may need to do outside of the main fix.
I set SIM applications>roaming> to manual and selected Virgin (I don’t see National but I’m on iOS 14 public beta 3 but I think it’ll work on iOS 13 too), dial assist set to off, then set mobile roaming to off and so far, touch wood, it seems to be working... worth a try.
PS I did try R101's suggestion but I couldn’t find the bit where he said select ‘national’ on my iPhone XR hence my option.
Well, I'm on day two without any corruption being experienced so my fix seems to be working. I hope others will try my suggestions (either on iOS 13 or 14 public beta) and send feedback on how they get on. I'm just happy that I've not needed to hard reset my iPhone on a daily basis now... Here's hoping the fix continues.