If this was a court-case, then your legal representatives may be able to get a statement from EE to confirm that your voicemail facility was disabled at the relevant time.
I sense from your post, that this is not the case, and it will probably just be a case of you telling them something to the effect of "I don't have voicemail on this number, so I suggest you are either lying or mistaken". And then seeing where the conversation goes.
