P-asserted header

A vendor requires a P-asserted header and I implemented as shown below. however, when doing a trace with Ethereal I do not see the header. I’ve got ethereal set up to filter port 5060 to get rid of the clutter. Anyone have any ideas?

exten => _XXXXXXXXXX,1,SipAddHeader(P-Asserted-Identity: <sip:${CALLERIDNUM}>) 
exten => _XXXXXXXXXX,2,Dial(SIP/${EXTEN}@${MAX-3662})