Call Waiting is a feature found on most business-grade VoIP handsets. This feature alerts you to a 2nd incoming call if you are already on an active phone call.
You will be able to place the first call on hold, or depending on your handset, you can also join the two calls together to make a 3-way call.
You can change the notification behaviour or disable this feature completely if desired.
Please see the relevant guide for your handset: