Re: Fines?
Switch to a system where it doesn't matter where the provider is.
Assign a short code. In the UK we have 1470 to mask one's own CLI and 1471 to give the number of the last incoming call. AFAIK the remaining 147n range is unused. Assign a number to one of them*. Dial that after a call and it's registered as probably problematic**. If the telecoms company recognises the source as problematic (and don't forget they do know where the call came from because they use it to bill the caller) then they credit the callee with a fee for receiving the call and add the fee and a handling charge to the bill. If the call originated with another telecoms company, even an overseas one, they bill that company. That company can pass the bill on with another handling charge added. Double (or more) the fee if the number called is on a do not call list.
In theory the victims get paid with credits against their phone bill and it's cost neutral at worst for the telecoms companies because their handling charge at least covers the costs unless the callers default. Defaults would be the equivalent of folding to avoid the fines. The telecoms companies have credit controllers so defaults will be limited. A telecoms company which doesn't keep proper track of the calls gets handed the bill and will change its ways PDQ or go out of business.
In practice, of course, this would kill the whole thing stone dead as the costs to the robocallers would go through the roof. The telecoms companies will realise this and know that if they're obliged to prepare for it they'll never get enough handling charges to cover their up-front costs. So proposing to enforce this will incentivise them to clamp down on the problem before the proposal gets taken any further. Once they're incentivised I'm quite sure they'd be very effective. On the principle that there should only be once chance to self-regulate any subsequent slackening off and the whole thing goes ahead.
* Vary for whatever numbering system works in your area.
** There's a risk of fraud if some toerag were to try responding to legitimate calls so the telecoms companies would need to gather a few reports from different people before actioning them.