Even verder ingedoken, zit nog wel een verschil bij cheapconnect dat het alleen UDP is (geen TCP). Dus de juiste manier om te checken is:
nmap -v -sU sip.cheapconnect.net -p 5060
nmap -v -sU sip.cheapconnect.net -p 5080
Dat geeft positief resultaat (ook achter het t-mobile modem)
maarten@mtwserv ~ % sudo nmap -v -sU sip.cheapconnect.net -p 5060
Starting Nmap 7.01 ( https://nmap.org ) at 2019-02-24 11:32 CET
Initiating Ping Scan at 11:32
Scanning sip.cheapconnect.net (185.29.203.27) [4 ports]
Completed Ping Scan at 11:32, 0.21s elapsed (1 total hosts)
Initiating Parallel DNS resolution of 1 host. at 11:32
Completed Parallel DNS resolution of 1 host. at 11:32, 0.00s elapsed
Initiating UDP Scan at 11:32
Scanning sip.cheapconnect.net (185.29.203.27) [1 port]
Completed UDP Scan at 11:32, 0.24s elapsed (1 total ports)
Nmap scan report for sip.cheapconnect.net (185.29.203.27)
Host is up (0.016s latency).
PORT STATE SERVICE
5060/udp open|filtered sip
Read data files from: /usr/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 0.66 seconds
Raw packets sent: 6 (208B) | Rcvd: 1 (28B)
maarten@mtwserv ~ %
maarten@mtwserv ~ % sudo nmap -v -sU sip.cheapconnect.net -p 5080
Starting Nmap 7.01 ( https://nmap.org ) at 2019-02-24 11:32 CET
Initiating Ping Scan at 11:32
Scanning sip.cheapconnect.net (185.29.203.27) [4 ports]
Completed Ping Scan at 11:32, 0.21s elapsed (1 total hosts)
Initiating Parallel DNS resolution of 1 host. at 11:32
Completed Parallel DNS resolution of 1 host. at 11:32, 0.01s elapsed
Initiating UDP Scan at 11:32
Scanning sip.cheapconnect.net (185.29.203.27) [1 port]
Completed UDP Scan at 11:32, 0.23s elapsed (1 total ports)
Nmap scan report for sip.cheapconnect.net (185.29.203.27)
Host is up (0.013s latency).
PORT STATE SERVICE
5080/udp open|filtered unknown
Read data files from: /usr/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 0.63 seconds
Raw packets sent: 6 (208B) | Rcvd: 1 (28B)