Sound like the dx_getxmitslot call is getting a bad handle in this case. I know I have used that demo on HMP in the past but it was still in context of HMP with DNI (for pstn), not SIP. I know it was originally written for both PSTN (JCT/DM3) and SIP (DMIP/PMAC) boards on SR 6.x baseline. But not sure how much more work was done on HMP as it was very old back then (as I recall) and the project source is probably just as old.
What actual line in the cfg file (outbound or inbound) are you running in this case?
Jeff M.