I had this problem several years ago and it has recurred.
My bill when I was in Croatia for a few days includes roaming charges for eight calls never made. All are to different UK mobile numbers in the 07953 range that belongs to EE. They vary in amount. I have tried all the numbers; none connect and they are not in my log so clearly I never called them.
I have called CS and got the charges removed but the person clearly did not really believe me and denied this had ever happened before. Well it has and it shocks me that you cannot fix it.
I'm really sorry to hear about these charges. This sounds very strange indeed! I'm going to have a chat with a colleague regarding this to see what we know about this. I'll be back in touch as soon as possible with further info.
I can confirm that in the case of the eight calls to my mobile (my first post above).
In the case of the other bill (my wife's) I don't know because she's abroad again now but as I mentioned, the bill does show incoming roaming calls at the same time. Would that be chargeable if they went to voicemail? And would that be shown in this opaque way on the bill? I hope not.