Playback audio to a FXS channel

I’m looking for some help to be able to play an audio file to an FXS channel that connects to an overhead paging system

Basically here is my setup:

INTERCOM = Zap/68

I tried to do the following but it did not work:

[playback]
exten => *7890,1,Dial(${INTERCOM})
exten => *7890,2,Playback(filename)
exten => *7890,3,Hangup()

Any help would be appreciated.

Thanks