As a young telecom engineer working in "Intelligent Networks" in the late 90's in the UK, I learnt that UK phones had a code that could be dialed during a call,

As a young telecom engineer working in "Intelligent Networks" in the late 90's in the UK, I learnt that UK phones had a code that could be dialed during a call, so that the line could not hang up anymore. That was geared against telemarketers or abusers, so that the callee could fetch a police officer that would acknowledge the fraudulent call. Today all this data exists in databases. It would be so easy to check call logs from telemarketers to check if they ever called someone on a do-not-call list.