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.