To help customers troubleshoot VoIP, I was thinking of using www.nmap-online.com to scan their NAT router to check that their SIP client successfully punched the SIP/RTP ports open on their router.
As a test, I used Nmap Online to scan my own PC which is behind a NAT router, but although UDP5060 was not used, nmap still reported it being "open|filtered":
"-sU -p 5060 1.2.3.4"
PORT STATE SERVICE
5060/udp open|filtered sip
Does it make sense? Should I use another online tool?