Issues sending SMS to Guernsey Channel Islands 07781 numbers

[trob001]

I'm having an odd issue with Guernsey 07781 mobile numbers.

Specifically I can RECEIVE sms messages from them, but anything I SEND is never received.

I know it's not an issue with the other number as I am chatting quite happily on an EE mobile via SMS.

I've tried all the obvious things, restarting phone, switching between RCS and SMS; changing the number to a +44 at my end, and at their end.

This has been ongoing for weeks.

I see there were some issues between JT & Sure and Sky and BT a few years ago but I've not seen anything recently.

Does anyone have any ideas of things to try, or have a similar experience?

Thanks in advance for any help.