EPT uses the Thomson ST 585 as ADSL Modem for their IPTV offer. EPT loads a special configuration onto the Thomson, which splits the 4 port Ethernet switch into 2 partitions. Ports 3&4 are bridged to an extra ATM path and channel (VCI/VPI) which gives access to the IPTV platform and network. Ports 1&2 give access to the Internet, by default the Thomson connects via PPPoE to the Internet.
The Thomson usually also supports a bridging mode which can be used to let a different device setup the PPPoE connection, e.g. a VPN Router necessary to connect to the office LAN, a VoIP device like a Fritz!Box etc. Unfortunately, due to the special configuration loaded onto the Thomson by EPT, this option is no longer available in the configuration interface of the Thomson.
At this point I almost gave up, but then I just tried to open a PPPoE connection from my PC, which worked out of the box. It seems that as soon as an other device opens a PPPoE the Thomson drops its own PPPoE connection and disables the DHCP server. This comes handy, as EPT has locked the configuration and the Thomson doesn’t seem to be very SIP friendly. With the new setup, the Fritz!Box takes care of traffic shaping and the SIP registration is much more stable.