There is no reason to allow ringless voicemail to telemarketers. For
metered plans, they still use up a consumer's minutes when they check
their voicemail and delete the unwanted message. They use the
consumer's time. They provide nothing in return for this.

Junk mail, at least before e-commerce started dumping gigatonnes of
parcel business into the USPS, subsidized regular post. Ringless
voicemail provides no such positive externality and there is no reason
to allow it.