Many thanks for your replies.
I'm quite out of my depth where mobile technology is concerned, but after a quick search, I think I understand what USSD codes are - in principle anyway.
I think that the Browser thought it is a phone number and if Number Validator is chosen, AMS will check if it is not an USSD code and if so, it will block it. This is to protect your device from being, for example, wiped by a webpage.
But what will the phone number (or code) be checked against? a pre-existing list in the program? or a list on the "network".
I saw this pop up a few days ago when I was copying and pasting an engine part number from a Google search. The part number just happened to be the same length of digits as a phone number so I guess Avast thought this was actually a phone number? I think the Avast pop up asked if I wanted to check with "Number Validator".
Just curious because that is the only time I've seen the Number Validator activate.
For me, it was when "clicking" on a phone number received within a SMS.