A call-to-waiting room system is similar to a paging system. The main difference between them is that the waiting room call system generally has more than one console of microphones connected in parallel.
An example for this configuration might be the one used for hospital applications. A microphone console is placed in each office and some acoustic players cover the entire waiting room.
This way doctors can call their patients directly from their desk. In case of some call overlap the system always gives priority to the first one that has taken the line.
The most used microphone console in these cases is the GT-501.