How can I make sure my iPhone blocks SMS spam when I ask it to?

I have an iPhone SE 2 (iOS 15.5). I keep getting obviously spam texts from the same sender; they look like this: As can be seen, the sender is clearly obvious - FreeStuff. They appear to be conventional texts (SMS), not iMessages. Touching on the sender name, I then touch on the 'info' icon, and touch 'Block this Caller', and select 'Block Contact'. That seems to work. However, if I navigate out of this menu, and repeat the process, it appears not to be blocked. Indeed, I am still recieving the spam texts, despite the attempt at blocking. I've also reported them to my network provider (7726 in the UK), but of course, I assume that's only as effective as their network-side blocking. I would like to do the blocking on my phone.