I'm almost 100% positive its _sip._tls. its what I used in my testing with polycoms and snoms to direct the phone to use TLS.
/b
On Mar 6, 2010, at 10:24 AM, Madovsky wrote:
# NAPTR for SIP over TLS
example.com
NAPTR 20 100 "S" "SIPS+D2T" "" _sip
s
._tcp.example.com.