Using BroadCloud Call Waiting
Call Waiting allows you to put your current call on hold to answer another call. If you don't want to answer the new call, that's fine, just continue your current conversation and the second call will go to your voicemail, or wherever you have directed the system to send the call.
The attached document, BroadCloud PBX: Call Waiting, provides all the information you need to get the best from this feature.