Said it before, I'll say it again. What's needed is legislation to enable recipients to be able to dial a code, say in the 147x range to register a spam call. Once sufficient complaints have been made their telephone account is credited with a fee - more if the number is TPS registered, which is charged to the caller plus a handling fee. The call arrives from a different network - that network gets charged and can pass on its own handling fee and tough on them if they don't keep tabs on where the call comes from. Abuse would be limited PDQ by credit control of the caller's telecoms provider.
In practice the threat of this and the cost of developing the systems to set it up would be such that suddenly the insoluble problem of controlling it by the telecoms industry would suddenly be solved.