Can't get on line !!

[#1]

I can only get on line by dialling any number 1471 for instance and leaving the phone off the hook. As soon as I replace the phone I lose connection.
BT say the line is OK.  Wondered if anybody has experienced this problem.
Thanks

[#2]

Do you have tepelpone extentions in your home.
What kind of router or modem do you have?
sound like you might have a noisy piece of equipment on your  line or a faulty microfilter.

[#3]

Yes I have some extensions, I have tried disconnecting all of them and have also changed the filter.  My Modem is called Speedtouch. The problem started on Monday, yesterday it was all back to normal but today has returned with a vengeance.  At the moment I am working with my phone off the hook having dialled 1471 and everything is fine but the minute I hang up the phone I will lose connection.
Thanks for your reply 

[#4]

I suggest following up the suggestions in this Broadband Reports post.
I think the acronym NID means Network Interface Device, telephone master socket in our terminology.
Have you tried connecting your modem (using a microfilter) at the master socket, preferably the test socket behind the faceplate if the socket is the NT5 type? That would completely eliminate your internal wiring as an issue with this fault.
Since you've tried swapping microfilters and disconnecting other devices it looks like a line fault is the most likely cause; try the quiet line test 17070 option 2 and listen - there should be no noises on the line. If there are there definitely is a line fault.

[#5]

Does it work if you simply unplug the phone?
A suggested plug into the test socket try without a filter as a modem doesnt need one (thats with no phones plug in)

[#6]

Many thanks for your suggestions,  I have switched on this morning at 7 am and everything is back to normal as it was on Tuesday of this week, most bl**dy frustrating.
I will note all that you have said and implement if necessary, fingers crossed that it now remains OK.
I do not have a great deal of computer knowledge so yet again many thanks for your help.

[#7]

Hi Speckledhen,
Glad to hear you're up and running again.
At a guess I'd say that BT were probably doing some maintenance work on either your line or exchange.