Can’t text one specific number

there is a specific number that I am no longer able to text, I can call the number and can receive texts from it but it doesn’t receive mine now (it did up to a few days ago)

i’ve deleted the contact details as well as deleting all text & call history to & from the number, switched phone off then on again & nothing helps.

The number I want to text is not an iPhone and I’m not getting anything to say ‘not sent’

anyone got any ideas?