This is how my door works. You lift the handle to engage the bolts in the doorframe (otherwise the lock doesn’t even turn), then you can turn it either a full rotation, and pull it out. The slit will point downwards and only people with proper keys can access. If you turn it slightly more, so it’s at an angle, then it’s in “service mode” and people with service keys can also access it.
You can’t put it in service mode from the inside.
If it’s fully locked and you want access for some reason, you’ll have to call for a locksmith. Alternatively remove the entire doorframe from the wall. It’s reinforced though so that’s going to be a hassle. The door itself is some kind of thick metal. Great soundproofing.
