Phone dead, ADSL works perfectly

[#1]

Since this morning I have had no dial tone and can not receive incoming calls (caller hears ringing tone but phone doesn't ring). 
I have individually swapped out the filter, the phone, and even the phone cable.
My ADSL is still working perfectly on the same line.
What on earth is going on?  How can the ADSL work and the landline not?

[#2]

Well,
What you are seeing is typical of either a single cable in the pair being disconnected (or significant degradation, it might as well be), or an exchange fault.
I've run a line test and found an exchange fault being detected. As such, I've raised it with our suppliers to investigate there.

[#3]

Thank you.  I imagine it is an exchange fault, but I shall remember what you said about one of the pair being degraded/disconnected, in case that happens to me in the future.