1. Keter
In general, this is a bad call. The object class is pretty much the last characteristic I determine when writing a skip, and not the first. Start with the story, build the anomaly, and once you know the anomaly you need to be able to tell the story you want, then you'll be able to say what the Object Class is. Starting by saying it's Keter basically means it's more important to be Keter than it is to… do anything else.
1 1/2. i was thinking that the person being called is SCP-XXXX-1
Unimportant, really. Definitely not worth determining first. There's no need to throw around extra numbers when you don't need to.
Story -> Anomaly -> Object Class and required designations for clinical tone.
7. was once thought of as harmless and not a threat
8. …until many personnel were found dead after being called by him. Cause of death unknown.
Why? Is it necessary to randomly kill people?
9. Hanging up = bad idea, why you may ask? Well that's never mentioned. mystery and suspense right???
Wrong. This is the wrong kind of suspense. Everyone is going to think, "Just hang up." So, when people don't, it's not mysterious, the victims are just stupid.
I'm sorry, I can't recommend the idea as written. The idea is being formed in the wrong order. It should start with why and how the calls are happening, and end with the specifics. And, more importantly, what you've got is something that just kills people at random and can't be stopped. That's not interersting. That's not engaging. There's no defense, no pattern, no reason to care. What choices can people make that matter? Furthermore, an entity that knows lots of information is utterly useless, if the only effect after the call is killing the called.