I have an ordinary land line through virgin. For months now when I pick up the phone the dial tone has disappeared. It is not the phone because the same thing happens when I plug in a simple old fashioned phone.
However, as soon as I dial in using my mobile the land line phone rings. The dial tone is now back and the phone works again. But after a while the dial tone disappears again.
Virgin's response was, predictably, bloody useless. The guy at the call centre said to wait until this happened again then not to call my landline first (to fix it), but to call them from my mobile. Yeh, right, so I'm expected to keep holding for a really long time on an expensive mobile call (my mobile isn't with Virgin - you don't put all your eggs in one basket).
My own thoughts are this might be some badly programmed software error. Maybe Virgin polls all the lines and drops any that show inactivity after a certain amount of time.
A web search will show that this kind of problem is by no means unique.
Rather than waiting for a customer to experience it yet again, the customer focused thing to do would be to send an email to all Virgin engineers asking them if they have seen something like this before. Assuming you pay enough to employ some decent well experienced ones, there is a chance one of them might know exactly what causes this kind of thing.
As someone who worked in comms for many years I find it extremely frustrating that I can only ever speak to the monkey, rather than the organ grinder.
I previously had a nasty broadband connection problem that I had to work out for myself. It was bloody difficult to isolate, but something that at least one of your experienced engineers WOULD likely have come across if Virgin was customer oriented enough to poll it's engineers about unresolved issues.