Despite what appears at first glance to be a giant empty hole in the books, they do end up being very precise in (nearly) all the right places.
What they do not tell you is when you don't have to be contactable: by definition, any time outside that when you have to be contactable, you are free.
A friend once described this concept to me as the difference between the Navy and the Air Force. He said that in the Navy, the regulations tell you what you cannot do, therefore everything else is ok. In the Air Force, the regs tell you what you can do, hence everything that is not specifically allowed is verbotten.
In the case of the paper under the door, who's to say the cleaning lady didn't throw it into the bin? Yes, you can quite legally (and morally) ignore any bits of paper or messages (even ACARS, although that gets harder...).
Vol 1, 2-3-6 para 7.24: "Reserve Duty A period during which the company places constraints on a crew member who would otherwise be off Duty".
Apart from this paragraph and the definitions of "Duty", "Duty Cycle" and "Duty Period" (where it would not be unreasonable to assume that you can be contactable!) you do not have to answer the phone, or open the door to a knock, or look at Crew Direct or Groupwise.
In short: a roster change can only be a two-way communication (phone or crewdirect acknowledgment) between you and crew control. Pretty much nothing else will do.