I want to replace my Broadband router (Speedport W 701V with integrated DSL modem) with a Ubuntu server equipped with a dual port network interface. Can I connect the server directly to the DSL filter? I guess the answer is no and I need a DSL modem to replace the router with my Ubuntu server, but I just want to be sure ...
Your hunch is correct: you cannot connect the phone line (with or without) a DSL filter to the Ethernet port. The phone line is incompatible with Ethernet at several levels.
If you want to connect to the phone line with an ADSL filter, you should connect to the phone modem port, which is often still built into laptops.
BTW when you connect to a filtered ADSL line, you get the low frequency, low bandwidth (4KHz) voice band. The ADSL data band is above 20KHz, which would be attenuated by the filter.