Made-up international roaming charges

I have now logged a dispute three times regarding international roaming charges for calls I did not make, and so far Cell C has failed to explain where those charges come from. When I activated my roaming the call agent assured me that as long as I don't answer an incoming call I won't be charged for it, and in order to prevent charges from callers leaving me voicemails I asked that my voicemail be deactivated.

At first, they said the charges were from callers leaving voicemails, and when I pointed out that my voicemail was deactivated the agent simply transferred me to their "rate your experience" survey and closed the dispute. The second time I logged the dispute they claimed that the charges came from outgoing calls from my phone, which is impossible, because a) I did not make ANY CALLS WHATSOEVER during the ENTIRE PERIOD that I was overseas, and b) in my bill it shows that the date, time and duration of the supposed "outgoing" calls is EXACTLY THE SAME as the missed calls that I received while overseas. So, according to them I was making outgoing calls at exactly the same time and for exactly the same duration as the incoming calls. It is obvious that those charges are for a voicemail service that should have been deactivated, but wasn't.

I logged a dispute for the third time, and despite the claim that they will get back to you within 48 hours it has now been a week with no further communication.