It is rather unfortunate but I believe it is an Openreach acronym for something like "Intelligent Solutions Information System".
Perhaps Black Sheep would be able to clarify?
I seem to recall that there is (or was) an Openreach SLAG Course ("Signage, Lighting And Guarding") for those working in public places.
There is definitely a 'SLAG course' .... attended a refresher myself, last year.
Now then, try as I might, I just can not find an answer to the ISIS question ?? I've typed into our powerful internal search engine all manner of questions .... including your own thesis of 'intelligent solutions information systems' ..... but it yields no results whatsoever ??
Being a nerd, I kind of pride myself on being clued-up with BT's abvs and TLA's .... but this one has me stumped.