When troubleshooting Zyxel routers that won't hang up, it's sometimes useful to analyze the packets that trigger the calls.
Menu 24.1 shows the packet like this:
LAN Packet Which Triggered Last Call: (Type: IP)
45 00 00 2C C2 00 40 00 7F 06 AA DD C0 A8 0C 7F C0 A8 01 1E 04 0C 00 87
00 05 07 89 00 00 00 00 60 02 20 00 DD 17 00 00 02 04 05 B4
This means very little to someone who doesn't read HEX, and even less to someone who doesn't know the specifics of the IP protocol. (i.e most of us...)