I didn't find it at all difficult to find when I left my spare 'phone on a train in Germany last year; the first time I've done such a stupid thing I might add.
Within a couple of minutes of realising what I'd done I'd reported and blocked it.
I don't recall having any difficulty at all in finding out how to do it and that was on another mobile.
@blackfive460 Clearly you were adept enough to find
Not all newbies would necessarily be able to.
I can't recall anyone telling me that I'm adept before...
From the front page, I went to the menu, clicked help, self help, typed 'lost sim' and the first result included the link. How simple does it need to be?
The problem is that there are many things that you could list that could be made more prominent but, for example, if you add them all to a menu, you are in danger of defeating the object of the exercise.
Losing a SIM is a pretty rare event so it's probably best to concentrate on providing easy access to the things that a brand new customer needs to do; topping up, buying products, porting in, that kind of thing. Hopefully, by the time a customer loses a SIM they'll have been here long enough to know their way around at least enough to take the simple option, as many seem to do, of asking the community despite the answer having been given many, many times before!