I did it on freepbx xxx many years ago, today I tried below steps on freepbx14, but not working.
creat /etc/asterisk/oss.conf :
autoanswer = yes
context = from-internal
overridecontext=yes
extension = s
language = en
playbackonly = yes
change /etc/asterisk/modules.conf : load = chan_oss.so
$chmod 0777 oss.conf
$chown asterisk:asterisk oss.conf
add below lines in /etc/asterisk/extensions_custom.conf
exten = *51,1,Dial(console/dsp,20,A(beep))
exten = *51,2,Hangup()
exten = *60,1,Answer
exten = *60,2,Playback(at-tone-time-exactly)
exten = *60,3,SayUnixTime(,IMp)
exten = *60,4,Playback(beep)
exten = *60,5,Hangup
$reboot
however still not working after reboot, always hangup when dialing *51