Re: Reporting calls
The terminating telco doesn't need to immediately block the route; they pass the info upstream and wait for the originating telco to cut off the subscriber. If the originating telco doesn't do so, then whoever's downstream of them has to cut them off. And ultimately, yes the terminating telco in the UK would have to cut off the inbound route, if the intermediate telco on that route has not dealt with nuisance calls upstream of them.
Clearly none of them are going to do this voluntarily, especially when they're all taking their cut from all these calls. International treaties and legislation will be required. Or at least the threat of legislation, if the telcos can't collectively get their house in order.
If it's ITU rules that are preventing anyone addressing this problem, then maybe the ITU is the organisation that should be tasked with solving it.