How to text Channel Islands

I know the Channel Islands aren't included in the inclusive allowance. Trying to text 07997 number in Jersey. I have credit and customer services says I can text premium numbers but it won't send. Anyone any ideas?

How much is your account topped up with.

£3.20

Have you tried 447997 in front of number

Yep, doing it as a +44 code and doesn't work...

Very strange are you sure number is right?

Yes, it's the MiL and worked fine until last April...
Ken

Is rcs chats enabled or just normal sms