Did you arrive at a solution/workaround, morddwyd?
I think I know why your text problem happens (though there isn’t an easy fix such as phone settings, I’m afraid). Is it a one-way problem (your son can text you but not vice versa)? Can you text both ways if you’re physically in the same country? And is the problem only between certain phone prefix combinations?
If so, it must be the same problem that started a year or two ago between me/spouse in Ireland and our daughter in the UK. It’s a network one, and any fix is down to one of the ISPs. (It’s the phone number prefixes that matter, regardless of which new ISPs the numbers may have been ported over to).
I delved into this problem in some depth (which ISPs don’t make easy), so I’ll explain it if you’re interested?
(P.s. Your thread’s still on the front page and I’ve only just noticed it isn’t very recent!).
Simple indeed, morddwyd - shame about the pain though! I was with O2 Ireland and, though it got subsumed into Three Ireland, I would have expected your charges to be roughly the same (less if anything, as everything’s more expensive in Ireland than in the UK). But our charge for a text to outside the EU is 35c.
Maybe you too have found out about the problem with some international texting that’s down to the vagaries of “mutual texting agreements” (or, just to make it more fun, whatever your partucular ISP decides to call them)? And this was their “solution” for you?
If so, perhaps you know that it's solvable if you or your son is willing to change to the right mobile network and use its phone number? (If it helps, I have a list somewhere of which UK networks have agreements with which ones (if any) in each country).