Now heres an interesting conundrum

My mother in law has one phone line and 3 extensions, together with ADSL from Talk Talk.

There are only three microfilters currently connected as one extension is a plug in type. The phone at the bottom of the stairs refuses to ring when the microfilter is connected, however, when plugged directly to the extension without the microfilter it does ring.

I swapped the microfilters over with the one upstairs and guess what? All phones now ring! I don't understand why..
If the microfilter was dodgy I thought that whatever phone was connected to it would not ring.

Broadband was also a bit dodgy, sometimes taking ages to open up a webpage, now however its working lovely.

Everything is about 20 years old, regarding the phone system.

There are microfilters connected at the master socket, and two of the extensions.

Hmm, very strange! It might be that the ring wire is not connected on the extension at the bottom of the stairs, but is connected on the socket upstairs... When using the possibly-dodgy microfilter on the bottom of the stairs phone I know it didn't ring, but did you get a good voice service to it at all?