The portal and mobile apps support two connected calls at the same time. While in a call you can initiate a new one. The first call will be put on hold, the second call will be active. Another case is that you receive second call during an active call. If you answer the first call will be put on hold.
The status of the calls – active or on hold, is shown on the top of the sidebar. Click on a call to switch between calls. The clicked call will become active and the other will be put on hold.