Mr Knowles told FTAdviser the PDG was looking into the issues surrounding such calls after a rise in the number of incidents reported to it.
The PDG formed in 2016 with plans to improve the insurance market for consumers as a ‘force of good for protection’.
Mr Knowles said: "The bigger distributors are coming across it quite a lot — we almost believe its some of these could be old PPI firms but we’re not sure.
"They’re usually calling from a dummy number which doesn’t ring through to a proper company.”
Mr Knowles said the issue was a slur on the industry as the callers were misselling protection and it was the customers who were losing out.
He said: "They’re being sold policies that are probably worse than their original one. The policies are then not going to pay out.
"I think we’ve got to be careful not to introduce fear-mongering because we do not want to paint insurers or advisers in a bad light due to some dodgy firms.
"The best thing for advisers to do is to have a good relationship with their clients and ensure to keep that relationship going so that, if the client does get the call from one of these firms, they will double check with their advisers."
Jiten Varsani, an adviser at London Money who has also experienced these calls and had noticed a rise in numbers since the start of the year, also thought there should be a ban on cold calling for protection alongside greater regulation and Financial Conduct Authority oversight to ensure that protection planning is treated with the importance it deserves.
He said: "The way these firms call up is scary because as an adviser we know so much better but for a consumer who may not know as much, they could quite easily make a detrimental switch.
"I’ve heard these callers often tell the consumer they need a review — which is worrying because that’s what we tell consumers when they take out the policy."
One example Mr Varsani gave was callers offering a terminal illness policy as a substitute for a critical illness policy.
Terminal illness cover is often included in life insurance as standard and will pay out a lump sum if the client's doctor has confirmed they have a terminal illness and are likely to die within 12 months.
Critical illness, however, is designed to cover serious health conditions from which policy holders might recover.
He added: "This does big damage later on when the policy doesn’t cover certain things or do the things the consumer thinks it does.
"The company then often disappears, leaving only the damage to the client and the industry."