None of these worked

; Zero call to check Jack - none of this worked
;exten => 0,1,Answer()
;exten => 0,n,JACK()
;exten => 0,n,Set(JACK_HOOK(manipulate)=on)
;exten => 0,n,Set(JACK_HOOK(manipulate,i(pure_data:input0)o(pure_data:output0))=on)
;exten => 0,n,Jack(o(pure_data_0)i(pure_data_0))

I checked that the jack module had been installed – change to astrisk src directory and make menuselect -it was installed correctly