I've been with Virgin Mobile for 6 years and I have never had this problem until moving to my flat at uni. I now can't get signal in or around my flat and don't get any until I get onto the uni campus. This means that I miss a lot of messages and phone calls, especially when I have a day off and don't leave the flat. However, I did have signal for around a week after I moved in, and will probably get signal for an hour a week inside the flat.
I don't know if there is anything that can be done about this, such as getting a signal booster, but I wanted to see if anyone had any ideas before I went into the shop to see if they can do anything.
The first thing is to check what signal other people on EE are getting, as Virgin Mobile is a "virtual" network piggybacking onto EE. That, and checking for faults on https://myaccount.ee.co.uk/networkchecker/checkservice will establish whether it's an issue with the mobile network (be it just poor service, or fault) or not.
If it turns out to be a network fault, then it's upto Virgin whether they're able to supply any temporary solutions, EE would offer WiFi calling and/or a Signal Box but I'm not familiar enough with Virgin to know if they offer anything similar.
But you first need to know if it's just your phone, or anyone using EE - and whether there's any known faults in your area.