Tasks like tell him about the dead person or he is staying in room 1963, call him

[Dark Messiah]

Who is this him? is it the same person? I feel like I somehow missed the name of the person I am supposed to call.

[DS]

The name of the person you need to call is not specified in this task, only the phone number - 1963. You are supposed to guess from other evidences who it is.

Hint: Yes, it is the same person. Multiple documents and the way this person talks give very clear hint who it is (there are not too many characters in the game).

Don't worry if you lack details right now, later you'll get more info to understand everything and reconstruct the picture, for now you just need to call the phone number to complete tasks.

In case you still need the answer (heavy spoiler): His name is Renzo Nero

[NishYou]

Just the Telephone book clearly trips anyone over along with these specific mental notes. Seeing all the 6 digit phone numbers in the book makes you assume you've missed a name while in fact the answer is just to call a 4 digit number.