Please help - phone line causing DSL connection problem

[#1]

Hello
I have both my internet and phoneline with Plus Net.
2 months ago we moved property and tool Plus Net with us, they moved the line for us remotely but no engineer ever came to the house.
Last nite we started getting DSL Connection Down errors when loggin on.
After much testing and research I discovered when I unpluged the phone the internet worked fine, when I put the phone line back in the green internet light went off on the router and the broadband light began flashing resulting in a DSL connection message.
I noticed when we first moved in the quality of the phone line when the internet was plugged as well wasn't very good, lots of hissing in the background. If I just plug the phone in and the internet out the phone tool works fine, no hissing. I have also tried other phones - same result.
So my question is how do I get my internet and phoneline working at the same time without any DSL conenction down errors or any hissing in the background of the phone?
Please help Plus Net!
Thanks
Chris

[#2]

This sounds like a faulty microfilter, do you have a spare one?

[#3]

Sounds to me like a dodgy/missing microfilter
I recall I did accidently plug in a phone without a microfilter on it at all into a spare phone point in the house here.
when I picked up the phone I got a hissing sound......and it really mucked up my broadband down from 8K sync to 2K sync.
I soon realised what I had done in seconds.
Note for the techy's
Yes the broadband frequencies are much higher than the voice ones but they can still saturate out the electronics in voice phone if they are not filtered out by the microfilter so you hear a hissing sound - you are not actually hearing the broadband signal itself - just the side effects of it on the phone.
Edit - previous poster got in first

[#4]

Yep, I concur with that. But, if there's a dodgy joint nearby though acting like a diode, you might hear some interesting noises then