asterisk dialplan example. person dialing hangs up the dialed number still rings and stays online for 7 seconds

I need a dialplan example which keeps ringing the called number even if the caller hangs up. And need to Keep channel open for 7 seconds by playing a soundfile. Or could transfer to a different context.. For example if a caller A dials caller B and caller A hangs up the phone before caller B can answer i want the phone to continue to ring and playback a soundfile..