A call / ring group is a place where you add your VoIP member/s, and or external numbers (eg a landline or mobile number)  which you want to ring when an incoming call is placed.  

All voip members or external members  in a call group will ring at the same time and the first to answer the call will receive it.

You can add as many additional call groups as you need, they are free and included in your monthly costs.  Using multiple groups, you can stagger the call flow.. Ie. Ring one group first, if no answer ring a different group.

Additional call groups are also used really well when using an IVR menu so that different phones are called depending on the option selected by the caller eg. Press 1 for sales rings only the Sales group