As someone working for a telco, not Vodafone, this would be my assumptions: A developer mistakenly grabbed a real MSISDN, instead of a QA one, while testing a promo still in development.
I only say this because there's no identifier to differenciate a real phone number from a test one. Subscribers often called to report those gibberish text messages they received. It's always a dev entering an incorrect number while testing.
When I worked for an Australian telco (not Vodafone), some developers on another team had used a very conspicuous mobile phone number in their integration tests, which actually connected to a real SMS service somewhere else in the company. No idea why they would do this. It turned out that this number belonged to a real person, who got absolutely buried in test SMS messages, when the integration tests ran as part of a CI/CD pipeline. The owner raised a complaint to the ombudsman, which led to all kinds of trouble for the developers.
In case anyone else here is curious, the ACMA maintains a list of reserved numbers for use in creative works, which you can use for dummy data: https://www.acma.gov.au/phone-numbers-use-tv-shows-films-and...
I worked at a grocery retailer, and we had the same exact thing. The CI/CD pipeline was firing out order related SMS messages to a contractor's number during test runs for years.
I wonder how common something like this is.
I was slightly more inclined to think it might be some bored employee somewhere acting in a sort of Robin Hood capacity just because it's unusually accurate and thorough for a test message. I'd expect more like TEST TEST test DFOIUHDFUOHDFOIUHDFROIHDSFOIHDSF LOREM IPSUM 999999.
Sometimes enthusiastic or particularly bored developers do put in the effort to write things out like a real message though.