Why does a call have two legs?

When a calls comes in from a customer and this is forwarded to your own number, there are two legs to the call. The inbound leg is the call from the customer to your virtual number, and the outbound leg is from your virtual number to your own number.  

 

A call does not always have two legs. If you have an IVR menu set up for instance, which does not forward to any other number, then the call only has an inbound leg.