BT phone out of action - but broadband keeps on working!

[#1]

I've just phoned in a voice fault on my BT line.
They tested it and they opine that there's a fault in an underground line.
How can they tell that the line is underground?
My broadband is still working (vast noise margin and only 4Mbps) even though my phone can't even recognise a dialled 151.
I find this a bit amazing.

[#2]

Does the phone work when broadband equipment is disconnected? Unlikely I know, but once had a fault where I could have phone or broadband but not both simultaneously.

[#3]

BT engineer called (at the door) within an hour of placing the call.
Apparently, "a pressurised underground cable" has failed.
Anyway, he put me on a different pair, went back to the exchange tested the phone.
Then came back to pick up some equipment + final testing.
The BT engineers I see are always very thorough. 
They will be fixing the underground cable "soon".

[#4]

Quote from: axisofevil

I find this a bit amazing.

Broadband will sometimes work (just) with only 1 wire. The phone definitely won't.
I know, I've done it.
(it's all to do with electrical properties at the different frequencies used)

[#5]

BT's quite sophisticated test equipment can tell how far from the exchange a fault is, and hence which bit of cable is affected.

[#6]

Does it have to be that sophisticated?They could tell where the breaks were in under sea telegraph cables many, many years ago!