P-Series Cloud PBX
Set up: Ring group with 3 extension, 201-203
Create a feature where each phone rings x seconds in sequence. Example; when a call comes in extension 201 rings on it's own for x seconds. If 201 is answered extension 202 starts to ring also(201 is still ringing). Again after x seconds 201 & 202 are unanswered then 203 begins to ring. At this stage all phones are ringing.