Load of rubbish that link, it was last updated in August 2017 when Virgin were using red SIMs. The issue is with the new purple SIMs which I think were introduced in December 2017 and we have all tried the troubleshooting steps that customer services have had us do with no success.
I have been having the exact same issues for a couple of months since moving to Virgin. The helpline is next to useless, the online chat has been offline for weeks saying no one is available and to check back during opening hours (this is during opening hours) and there is no email address to try either and finally managed to find this thread. Tried another sim from another network and everything works fine and my phone worked fine with o2 before moving over. Also most times I press voicemail I get a wrong number dialled message and only occasionally get connected. The phone gives no indication as to when the data isn't working properly and sometimes even some apps work with data while some others won't connect at all. Meaning that sometimes you've missed all sorts while you've been out, resulting in a torrent of work emails when you finally flick airplane mode on and off to reset the connection. Evidently this problem across the network needs addressing urgently as so many people are having identical issues and it seems there are a fair amount not receiving the service they are paying for.
I cant get my head around the fact that customer services don't recognise there being an issue because data is being used. This is the reason they told me they won't terminate my contract.
Because I have used data every month then that must mean that there isn't a problem with the network (this is what they are telling me). They don't understand that because the issue is intermittent data can still be used.