Some airlines have cabin procedures that relate to altitude: For example, Cabin Prep at TOD, Cabin Secure & Sterile Cockpit at 20,000ft and so on. These alerts can be generated by illuminating signs or by single or double chimes on the PA system. If there is a fault, or suspected fault with these systems then other methods need to be agreed. For this reason, many airlines will develop a procedure for a prepared emergency landing with P.A. failure.