Internet only works when I call Plusnet helpline

[#1]

Hi,
My parents have recently joined joined Plusnet. Everything worked fine for the first couple of weeks, but for the last five days they have been unable to connect to the internet unless they ring Plusnet to report the problem - Basically the internet connection springs to life when the call gets answered by Plusnet, then cuts off when the telephone call ends. Ringing another number has no effect.
Luckily the call answer time doesnt seem to have improved over the last few years so they get an hours worth of internet before someone picks up the call and tries a  load of stuff . As you can probably guess, nothing seems to work as of yet.
Has anyone else experienced a similar problem?

[#2]

Have they made sure all devices connected to the telephone line are filtered?

[#3]

yeah,
they've just got the router and phone plugged into the filter, nothing else. They've changed the filter as well and that had no effect. Strange thing is it works perfectly when they have an open phone line to Plusnet. As soon as the phone line closes, the internet goes off.

[#4]

Sounds like it could be a fault.
I'd recommend getting it raised by going here
https://faults.plus.net

[#5]

If the router only (i.e. no phone connected) is plugged in to the test socket inside the master socket does the internet work?