I get calls about Electricity & Telco's about switching providers.
With the electricity I ask them what their In-feed Tariff rate is as I'm a net producer of electricity (I wish) they usually hang up quickly after that.
As for telecoms companies I just make up some lowball prices on the line & call rates, or ask if they have fixed the problem that I reported.
Surprisingly a lot of them are quite rude and just hang up.
As for emails - if you are offering me money or telling me to claim something and I don't know you then it get deleted.
As do all notices from PayPay, eBay, Western Union, Spanish Lottery etc...