It can be very tempting to pop out some ceiling tiles to get a long phone cord from one side of a room to the other side, or from one
room to another; but it can also be very dangerous.
If the space above a ceiling, but below the floor above it, is a “plenum” — a space used for circulating air for the building's heating and cooling system — you should not just drop in a common PVC-insulated phone cord.
PVC (polyvinyl chloride) is a ubiquitous plastic that’s
also been used for toys, phonograph records, food containers, credit cards, house siding, water pipes, shower curtains and 1001 other things.
PVC is useful, plentiful and inexpensive, but it has one
serious problem. It can kill you.
When heated to high temperatures in a building fire, PVC
releases poisonous gas, forming toxic hydrochloric acid when inhaled by people who work in the building or by firefighters. Once this potential threat was realized, building codes were modified to minimize the potential
PVC insulated wire either has to be enclosed in protective
metal conduit, or PVC is avoided in favor of wire with heat-safe Teflon or similar insulation. Teflon is more expensive than PVC insulation, and a real PITA to work with (it’s hard to strip off and it can cut
installers’ fingers), but it’s less expensive than conduit, and doesn’t emit poisonous fumes in a fire.
Ordinary phone cords should not be put above a ceiling if the space between the ceiling and the floor above it is a plenum. If you are unsure, ask the building
owner or manager.
Photo shows "Classic Fine Textured" 24" x 24" tiles by Armstrong.