OFCOM rejected my complaint
Back in the days when UK telecoms mostly comprised BT and odd ball companies, before mobile telephony was a thing, I had a billing dispute with BT. I wrote to them; BT did not reply. And again.
I pursued this with OFCOM who wrote to me that I needed a letter from BT, the company who declined to write to me, in order for them to follow up. Thanks, OFCOM.
I wrote to BT telling them that I would stop paying my phone bill until the problem was resolved. One Saturday morning my phone line was cut off. I could still phone BT where I spoke to a pleasant operator who reconnected my phone on a temporary basis and contacted the billing department.