One that you believe in

The ascent of robo calls and the do not call list has pretty much ended cold calling. You’d need an opt-in list.

The medium is the message, any product that has affinity with the medium will have an advantage.

Phone plans, VoIP, phone answering services, cold-calling services, marketing.

Additionally any industry that relies on publications of buy or sell offers, like Real Estate or Staffing is fair game. If a hiring company, available candidate, house seller or buyer is serious, they will publish their phone numbers on multiple places and will be open to straight up phone calls without even questioning why you are calling them.

This rituals where buyers/sellers open themselves to contacting them makes them much more prone to cold-calling as opposed to some businessman who never gave you their phone number, but you somehow managed to scrape off the internet.