If you just need to start an instant call with one of your Biba contacts, click the call button from the launchpad.
This will bring up a pick list of your contacts, which also shows their current availability.
Select who you want to call.
You will see the call roster with you showing as present and your callee showing as invited.
Your callee will receive a new call alert, which they can answer, decline or select a time that they're running late.
When they answer, you will see their status change to Present on the call roster.
The important differences between a Call and a Meeting is that a call initially only lets you add one other Biba user to the call, and it starts locked, so outside Biba users aren't able to dial in until the organizer unlocks the call.