Can't upgrade or cancel a contract

I first app**** for an upgrade on 09/07/2021, after which I have called in about every two days since then. The first few times my application was on hold, with no communication about it given through. Every time I was told it was resolved and the courier would contact me. The last time I was assured delivery would happen by 30/07. No contact from a courier, so I phoned in again only to be told my order was cancelled, as the phone was out of stock. I tried to escalate, no response. I tried to cancel my contract but can't even reach the retentions department, I was on hold for 45 minutes before giving up.

Such a pity as I was a loyal customer for over a decade.