Text Support Not Working

[#1]

Does anyone else find that the text support simply does not work?
I tried in August and again today.
It just keeps saying text could not be sent.
My text messaging works as normal for literally every other phone number. Its as if they've blocked me.
Not helpful in the slightest

[#2]

If it says it 'could not be sent' that implies a problem with your phone, otherwise it would be sent.

What are you trying to resolve?

[#3]

The exact wording is
! Not sent. Check options.

The options are resend or delete.

The number I am texting help to is 07800 008121
Tried deleting it as a contact and adding again.
Made sure my mobile number is linked to my account. Tried using mobile network to text instead of WiFi.
I have successfully used this service abkut 18 months ago from the same mobile number. Its as if I'm blacklisted.
As I said, texts to and from everyone else are working as usual.

[#4]

I've tried clearing message app cache, force stop and restarting the app, restarting phone

[#5]

Extremely strange - I think I may have seen this error a couple of times in the dim and distant past, but can't recall a resolution - if there was one.