While trying to improve our Adsl connection, I found that we use a splitter that is from an unknown ventor and have no clue whether is for AnnexA (PSTN) or AnnexB (ISDN) line.

Is it possible that somebody used the wrong splitter and just because it connected nobody ever cared?

We have an ISDN line but the splitter used is blank. I am not really sure if it is AnnexB. Would it be possible to use an AnnexA filter over an ISDN line (even with that much signal loss)?

(I am posting here because this is about our connection to the rack, I'm not sure if there is a more proper SExchange site)