telegraph is usually a visual(can be sound based) pre-cast warning that signifies enemy ranged/AoE attacks, highlighting areas where attack will land or direction of attack.
i.e. it sends advance warning.
Example visual:
Player A casts meteor on player B.
Players B sees Player A pre-cast animation and circle area around himself become engulfed with fire(typically with no or minimal damage), Player B flees, meteor slams into the fire circle, causing no damage(except terrain damage over time).

Example sound:
Player A casts invisibility spell.
Player B hears a specific sound associated with Insivibility (and possibly last location of player A is highlighted in red). Player B flees the area until invisibility expires.