I received this type of call about 2 weeks ago. The call I received it was technically voluntary, but heavily biased toward making you accept because if you hung up on the call, you were enrolled in paperless billing. After the initial description of the benefits of paperless billing, it said something like this:
To enroll in paperless billing either hang up or press 1 now, if you wish to keep receiving bills in the mail press 2 now.
I bet a lot of people just hang up before it ever gets that far. In that case they'd be switched to paperless billing.