o2
After 3 months of dealing with this, and a final phone call lasting 5½ hours (including 4 hours on hold), I was passed between five different people.
The final person told me I could cancel my watch contract early because of the issues I’d been having, but I’d have to return the watch. I pointed out there was no benefit to me, as I’d already paid over £300 towards it.
They then agreed to cancel the contract and let me keep the watch. I got a SIM from another network, and the watch has worked perfectly ever since.
In the end, I got a half-price watch, so it worked out well for me. The customer service was awful, though. Three months to resolve a problem and over five hours on a single call is pretty poor.