I use an iSavi IsatHub for Internet connectivity when I'm in the backcountry out of cellular service. This device is a BGAN terminal and it works by establishing a data link with an Inmarsat satellite and then provides a WiFi network to connect your devices.
Data usage is very expensive, so it offers a number of safeguards to prevent accidental usage. One of those safeguards is a firewall.
I only need SSH connectivity, so I blocked all outgoing ports with the exception of TCP port 22, UDP 53 and TCP 53. All inbound traffic is allowed.
This past week I was in the backcountry far from cell service. I fired up the IsatHub and connected my phone to the WiFi network. Much to my surprise, I started receiving text messages to my Verizon phone. More to my surprise, I was able to reply to those text messages and have a back and forth conversation. The satellite terminal has its own SIM card and SMS capabilities, but these messages were received over my Verizon SIM.
I've searched the net for any documentation on network ports used for WiFi texting and calling. I found this and it does include TCP53 and UDP53 (which I assume are for DNS lookups), but it also includes UDP500 and UDP4500 which are standard IPSEC VPN ports.
Does anyone have an idea of how this was possible? How the heck was I able to send text messages over WiFi with my Verizon phone when I've blocked all outgoing ports except those needed for SSH and DNS? Is it somehow related to accepting all inbound traffic? Could Verizon be re-appropriating port 53 for WiFi texting?
Edit: When I returned home, I connected my phone (airplane mode, wifi turned on) to my home WiFi network, sent a test text message and took a capture of the network ports in use. The only ports I saw in use between my phone and Verizon-owned IP addresses were ports UDP500, UDP4500 and TCP443 (500 and 4500 being used for the IPSEC VPN ports I mentioned earlier).