Zaptel and Call Waiting woes

I have calls coming into my analog line via a digium card and then I Dial a Sip Extension.

I have callwaiting enabled in the zapata.conf and I hear the beep then another call is comign thru. I click flash and then I read somewhere that *0 should send a Hook Flash. I just get a fast busy. I click flash again and it goes back to the last call.

Am I missing a config somewhere to get the callwaiting to work on the zaptel device ?

thanks !