Some great advice in this article on how to avoid cold callers, here are my tips:-

1. Don’t answer numbers you don’t know on your phone.
2. Most cold callers won’t leave a voice mail. If calls is important then people will leave a voice mail.
3. If you get a cold call use the number blocking facility in the phone. On iPhones next to the number is a ! in a circle, press that and at the bottom is “block the caller”.  If you start using this the number of cold calls should start to reduce.
4. You can ask the caller when they call you or you can do this through their website to invoke your “right to remain forgotten”. As a European citizen, people have to comply with these GDPR rules, regardless of where they are in the world. This means that the company has to remove you from every system, database, CRM, salespersons spreadsheet or risk a fine of 4% of annual turnover. It’s critical you keep an audit trail of requests so you can submit a fine request to the ICO.
5. If you do get a call, the key thing is to waste the callers time, they hate it. Cold callers are paid to get on and off a call, as quick as they can, selling you something on the way. You can do this by being silent, putting your phone down and walking off, or I’ve even barking like a dog. Anything to waste the callers time.
6. Don't ever give your details, passwords etc out over the phone even if the caller says they are a trusted brand.