====== call ======
line.call contains [[ixnx:wrtc:js:api:call:start|call]] object using this line, if any. Each "telephone line" can carry one call.

===== Default value =====
undefined

===== Value =====
  * [[ixnx:wrtc:js:api:call:start|call]] object if line is used by a call
  * undefined or null if line is not associated to a call but is free to receive or make new call