Phone companies at least in the US are taking initiatives to screen spam callers, but there is only so much you can do when there are untrusted actors on the network. Truly ending robocalls would require a global treaty encompassing both hemispheres, and probably the creation of some new UN agency responsible for registering network endpoints (like ICANN). As it stands you can just move to another country that lets you hook up to their infrastructure if you are ever kicked out of your current base of operations. Given that there are still countries where you can e.g. host content that is illegal in the US or other countries, it seems unlikely such a treaty would ever make it into international law.

Originally it wasn't a thing to be able to transfer phone numbers between companies if you wanted to switch providers.

The industry pushed back hard saying it was too complicated, would cost them too much in development time, etc. Govt. still pushed and got it through, and now we have more control over our numbers.

If they're fined significantly for forwarding spam, then you bet they'll try to find a way to stop it.

Untrusted / unauthenticated does not mean not traceable. You don't need an international agreement to tell companies in your country "you're responsible for not forwarding spam calls, but you also don't have to accept calls from peers with bad spam ratio". Telcos know where the spam comes from either directly or indirectly. They just don't have a good enough incentive to drop partners.

Spam calls are simply not a problem in many countries outside the USA, so it must be possible to tackle the problem without a global treaty.