Call centers and oncall are different. Oncall is "when Facebook suddenly stops working, who is responsible for figuring out why and fixing it". These systems of

Call centers and oncall are different. Oncall is "when Facebook suddenly stops working, who is responsible for figuring out why and fixing it". These systems often have SLAs (either internal or public) in minutes. Response (ack and log on) times for these can be 30 minutes for less relevant services, but some have 15, or even 1-5 minute response times for highly critical stuff.