Each industry has its own parlance and magic words. In telecommunication, whatever the underlying technology (SIP, WebRTC, TDM, etc), you will very often read about call "legs" and "channels".
First confusing fact: each "call leg" is actually a call in its own right. That is: a call is often said to be made by an "A-leg" and a "B-leg". In fact, "A-leg" and "B-leg" are proper calls.
Second confusing fact: each leg is a channel. So, most of the "calls" are consisting of two channels(A-leg and B-leg), while some (to IVRs, voicemail, and the like) are consisting of one only channel (A-leg).
The reason for this funny terminology is the fact that when people talk about "a call", they usually mean an end-to-end voice or video connection, from the caller to the callee. Also, and maybe more importantly, calls were billed this way, as a complete "circuit" established between caller and callee.
Such a complete "call" circuit, when you have a server in the middle (for example, FreeSWITCH...