If I can just say, it shouldn't ever be a case of the customer's word against VM's. All telcos are required by Ofcom to keep records of all re-contract discussions and retain those for a minimum of six months.
If needed VM can be forced to disgorge the relevant call recordings or chat/Whatsapp transcripts by a Data Subject Access Request. If they can't or won't provide those records, complain to both Ofcom and the ICO.
