Configuring VOIP (inode xDSL - Patton Smartnode - Agfeo AC12 USB)

A few days ago I had to configure the following setup:

Internet Provider: inode
Internet Modem: Zyxel Prestige 700series G.SHDSL modem
VOIP Gateway Router: Patton Smartnode 4552
Telephone Switchboard: Agfeo AC 12 USB

The VOIP setup didn't work at all, so I first checked if there is any DHCP server around in the LAN. I deactivated the DHCP server (it wasn't used anyway, all clients had designated IP addresses) and the VOIP Gateway was online the first time (FYI: Most/All(?) inode VOIP gateways are configured to get a DHCP address from the provider and if you have a local DHCP server that does not support excluding MAC addresses from automatic address configuration you have to turn off your local DHCP server).

Afterwards I figured out, that some of the cables weren't connected to the correct sockets, so here's an overview of the cabling setup:

POTS@wall                      -> Line@Zyxel_Prestige
LAN@Zyxel_Prestige             -> LAN@Patton_Smartnode
Phone(BRI0/1)@Patton_Smartnode -> ISDN_S0@Agfeo_AC12

Afterwards, the Phone and LAN leds on the Patton Smartnode should indicate the correct link status.

Now I was sure, that I correctly connected and configured my LAN but it still didn't work.
So I called inode and they told me to call Agfeo and ask them if they need Point-to-Point or Point-to-Multipoint Encapsulation on the ISDN line. And that was it. I called Agfeo (1,81€/min!!) and got the information, that the Agfeo AC12 USB only works if configured with Point-to-Multipoint.
Another call to inode followed where they switched the encapsulation to the correct protocol and one reboot later, everything worked fine.

Well, maybe that doesn't sound very interesting, but I spent nearly three hours to fix this problem, because before knowing anything about those encapsulation protocols and their relevance in terms of VOIP I presumed the misconfiguration to be on the Agfeo TK suite and tried to track down the error on this side...

Note to myself: Learn some internals about VOIP...