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Place a call, retrying on failure allowing optional exit extension.

  RetryDial(announce|sleep|retries|dialargs): This application will attempt to
place a call using the normal Dial application. If no channel can be reached,
the 'announce' file will be played. Then, it will wait 'sleep' number of
seconds before retying the call. After 'retires' number of attempts, the
calling channel will continue at the next priority in the dialplan. If the
'retries' setting is set to 0, this application will retry endlessly.
  While waiting to retry a call, a 1 digit extension may be dialed. If that
extension exists in either the context defined in ${EXITCONTEXT} or the current
one, The call will jump to that extension immediately.
  The 'dialargs' are specified in the same format that arguments are provided
to the Dial application.