I have no idea how a GUM clinic functions when they return a positive test, except in that many, many years ago an ex girlfriend had returned a positie test and they asked her to contact all her sexual partners from recent years to come in for tests. She did, I attended and was negative.
Could it be that if she didn't want to do that, would/could the GUM clinic collect data from her and pursue the matter?
I'd check the number on the Internet;
whocallsme.com
Possible post it here.
I'd get checked out via my usual GUM clinic.
Phone the alleged clinic and ask if the do send texts out, if they do, how did they get your number?