PRI_CAUSE for zaptel with zaphfc-s card


im trying to signalling a busy-Tone to my ZAP-Channel if the called numer is busy.

This all workes well to the point where i try to signaling a busy tone to my ZAP_Channel.

Here is my script to do this like described on

exten => s-BUSY,n,Set(PRI_CAUSE=17)
exten => s-BUSY,n,Hangup()

This lines should indicated the ZAP-LIne to singal BUSY but the PRI_CAUSE is ignored and the line is hungup without an busy signal.
Does someone has experiences how i can set this correctly. Does the hungup-Application ignore the PRI_CAUSE and needs an HANGUPCAUSE??