SMS messages after update to 11.1.1

I updated my iphone 5s (in the UK, on E.E.) to v11.1.1 yesterday and ever since I have been plagued by pop-up "SMS" messages that read: "Sorry, the number you are sending to does not contain enough digits. Please check the number and try again"

I have tried all the suggestions I can find on the EE forums: imessage off/on, facebook off/on, wi-fi calling off/on, reset network settings, reset all settings, use iTunes to restore the phone to new, use icloud to restore the phone to new. No change - the first thing that happened when I reconnected to the wifi network was another SMS came through. (I do not have a mobile signal in this area so everything is through WIFI calling) .

I have spoken to Apple support, who advised me to do all the above, (again) which I have done, but the first thing that happened, again, was one of these pesky SMS came through. Anybody any ideas? Thanks