Does the Virgin telephone service test ever work? We frequently have a problem which results in incoming call attempts resulting in the message 'Number Not Recognised - please check and redial' and outgoing call attempts producing the engaged tone as soon as the phone is picked up. However, running the phone test indicates that the line is working ok. We also had a problem of a noisy line on outgoing calls, and incoming calls being terminated after one ring, but again the test reported that the line was working ok.
I'm really sorry to hear that you've been having quite a few issues with your phone line, and that you've been unable to see a fault when you've run the line checks.
I've just run some testing on the line, and can see some issues that will require a technician to call out and investigate further. The on line test certainly does work, as I've seen several customer's book technician's using this method in the past. However as you mention that your fault seems to be intermittent, it could be that the issue I'm detecting is also intermittent, and I've been lucky to catch it whilst it's happening.
I've sent you a private message in order to get things arranged with a technician booking, please look to the top right of this page where you'll see a purple envelope icon, click on this and you'll see my message.