I'd imagine there's virtually no cases of consumers and businesses voluntarily using arbitration when there was no contractual agreement. Both sides need to agree to use arbitration.

I can dig through the data in a bit: they have the info under "source of authority", which will say whether it was in the contract or agreed upon later.

Looking at att in particular, they settle most of their cases. https://levelplayingfield.io/party/non-consumer-att