# Calls and lines

In the ADI service call control model, a call represents a connection between two parties (or a connection in the making, or a former connection), and the trunk channel on which the connection resides. The states of both the channel and the connection are included in the same state machine.

The NCC service call control model introduces the concept of a line, as separate from a call:

- A line is a logical representation of the channel on a trunk. It is referenced programmatically using a separate handle, called a line handle (equivalent to the context handle).

- A call is a connection between two parties (or a connection in the making, or a former connection) on a line. Multiple calls can exist simultaneously on a line. However, only one call at a time may be active; that is, not disconnected or held. A call is referenced using a call handle. When a call control function is invoked, in most cases a call handle (and not a line handle) is explicitly referenced. The line handle is referenced implicitly.

When a line-specific event occurs, the event indication includes the line handle. When a call-specific event occurs, the event indication includes both a line handle and call handle.