It's not virgin's bad signal, rather the EE signal that they piggyback on as an MVNO. The only difference for signal strength/quality is the missing 800mhz frequency that EE's own customers have. That is used for 4g calling anyway, which along with WiFi calling virgin don't have.
Virgin won't be able to wave a magic wand, all they can do is check with EE. As you said it happens wherever you go, that would be more difficult anyway.
Looking at the signal maps you should be getting a decent signal where you are. As I don't have an iPhone, I don't know if there are signal strength apps you can install to compare 2g/3g/4g.
It could be the case that one or more of those gives a better signal at home for calls. If you have WiFi you don't really need 4g at home, as calls are made on 3g or 2g. I move to 2g when at home, as my 3g signal for calls is bad.
Not sure from your posts if you said you'd tried your sim in another phone. You mentioned others with vm have issues, is that tested on 4g,3g,2g?
Entirely possible that you have a faulty sim. I know its a replacement, but I'm pretty sure that's happened before on this forum.