Crackly phone line, but stable Fibre

[#1]

Can anyone explain how I'm experiencing intermittent crackly phone lines, but the fibre connection is rock solid?
We've noticed crackling on the (PN) phone line getting worse over the last 3 weeks or so (to the point where we can't hear the caller and vice-versa), but the fibre connection has not dropped once or suffered speed loss. The noise sometimes goes away, although in most cases re-appears after a day or two. This is bizarre as I always thought a noisy crackly phone line would affect the broadband signal, and thereby kick in DLM?
Any advise greatly appreciated.

[#2]

If the fault is between the cabinet and the exchange then fibre wouldn't be affected as on fibre the copper cable is only used for the connection between the cabinet and your property.

[#3]

Given that the broadband connection to the exchange is via the fibre cab and a fibre link whereas the phone line uses the copper connection from the cab to the exchange then that points to the problem being on the cab exchange connection
Edit - as Matt said

[#4]

Thanks guys. How would the engineers fix this then?
Cheers!

[#5]

Any ideas on how the engineers might go about fixing this, if it is a problem between the cab and exchange? Could they do it during a normal residential cal out?

[#6]

Interesting. Same problem here today. Terrible crackling on the line but no issues at all with the broadband connection.
I didn't realise that when you were switched to fibre the copper was still used for the voice part of the cabinet/exchange connection - just assumed it was digitised and sent down the fibre!

[#7]

@artfulbodger
The engineer can still work on the copper side of things to repair your phone line.
@rogerx1
Have you tried following the steps on the link below?
http://www.plus.net/support/phone/troubleshooting.shtml

[#8]

Guys, many MANY thanks for all the advise.
Engineer visited last week - couldn't see anything wrong via his tester at the master socket inside the house, and all other cabling was fine. However when he visited the cabinet he found a "loose connection" which he fixed and everything works beautifully with the phone now (absolutely crystal clear line!).
Straight after he had sorted everything out my profile was still the same rock solid 80/20 (or 77.4) that it's always been since I moved over from BT to PN. However a few days ago I noticed a slight decrease in performance, and saw that the profile had marginally dropped. When I phoned the helpdesk they advised restarting router and modem - which resulted in no increase at all back to the 80/20 as before. Also, bizarrely, for a week after the engineer's visit my usage stats showed zero broadband activity - even though we were hammering it like mad as per usual.
Can anyone advise what's going on? How can the connection/profile drop speed simply after getting the phone line fixed? Also he didn't carry out or even offer a line reset?

[#9]

As it was a phone engineer they wouldn't reset the fibre connection as they aren't a broadband engineer.
In terms of the usage if it hasn't shown up I wouldn't worry too much about it as your on a Unlimited package.
The profile may have dropped because of the work he did or another engineer working at the cabinet.
All in all everything looks fine.

[#10]

Matthew I think it was another engineer as just after he had left my connection was still perfect. Is there anything that can be done to find out?

[#11]

My line/modem stats are as follows - are these ok?
Line rate: 79475 / 19999
Attainable rate: 79024 / 27543
SNR: 9.7
Attenuation: 12.5 / -5.3
HEC: 186 / 4294967290
CRC: 254 / 127
FEC: 0 / 0

[#12]

Unfortunately we can't I'm afraid.
Those stats look fine.