|Hi Sebastian, am based in London, UK.|
Have just this week cracked a obscure similar problem, where some (for testing) phones on an extension would ring, but the original phone would not.
Eventually traced cause to:-
i) some phones/devices use only pins 2 and 5 for ringing *AND* speech.
ii) other phones/devices *ADDITIONALLY* require/use pin 3 for the ringing.
iii) pin 4 is not used, but generally connected
The phone socket on the broadband filter only had:-
pins 2 and 5 present
pins 3 and 4 were missing - crazy!
Changed to a filter with pins 2 - 5 (metered to confirm all connected) and everything worked.
Please advise back on progress.
Good Luck - Keep us posted.