Your phone lets you handle multiple calls at the same time. Whilst you can only have one call connected (talking) at any time, you can have other calls on hold.
A slow flashing red lamp on an appearance button indicates an alerting call. For additional calls, there is no ringing or just a single alert tone.
If you are already on a call, answering a new call automatically put that existing call on hold unless your first press Drop.
Note: This guide assumes that auto hold is enabled (the default). The system administrator can disable auto hold for the system, in which case calls are automatically disconnected rather than put on hold.