Callblock of Duty is the widespread phenomenon of rarely being able to reach someone via phone while they are gaming, regardless of their choice of console (or PC) and irrespective of which game they may be playing. It is, however, particularly prevalent in serious Xbox gamers, specifically those who frequently play one or more of the Call of Duty game series. Your call will most likely ring all the way through to voicemail, as even the simple act of hitting the "Ignore" key on a cell phone might break said gamer's concentration and/or compromise their strategy/position. If your call goes directly to voicemail, said gamer has most likely already received and declined to answer several calls, triggering the automatic voicemail response feature of most cellular carriers, or he/she has actually turned the phone off to avoid further interruption.
A: "Hey, is Matt coming or not? We've gotta leave soon or we're going to miss the movie!"
B: "Probably not. He must have activated Callblock of Duty; I can't get him on the phone. We should just go... He'll call me back in like 6 hours."