Text from US to UK

[CarryOn]

Hi - I’m attempting to text a UK number from my US-based iPhone. I’ve scoured the internet and this board, but I can’t get a consistent answer, and no matter how I input the UK number, the text is never received. 

I do have an unlimited plan with international messaging, so it’s not that. These are the formats I’ve tried so far, based on what I can find online (including here):

The number I’m trying to text is like this:  

079XXXXXXXX

I’ve tried:

011 44 079XXXXXXXX
011 +44 079XXXXXXXX
+44 079XXXXXXXX
011 44 79XXXXXXXX

Does anyone know for certain what the correct format is?

Thanks