impressively convoluted. Spam calls are coming from all over the world using voip and prepaid sims. There aren't any accounts to "bill". Not to mention a whole new arena for fraud in spoofing regular numbers for micropayments
To end up on your phone, VOIP have to terminate somewhere in local phone network. These networks need to stop accepting calls from shady places, or every call like this that they cannot charge back to original (or intermediary network) becomes a loss to them. And telecom companies do not like losses...