Try this. Cured it for many
Go to:
Settings -> mobile data -> SIM Applications -> tap "Roaming" -> tap "Select Mode"
Im now trying this
I found "automatic" was selected by default.
Tap manual -> then choose National.
Now make sure your numbers are set to the correct format. If not change region to anything other than UK (Settings -> General -> Language and Region -> Region) and then change back to UK region
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