Mr Dewfall, of Trowbridge, asked in last week’s letters page, “there must be some way to block [unwanted] phone numbers.”

Well, there is and it does not rely on ineffective notification to your provider or registration with TPS.

I installed a True Call call blocker on my elderly parents’ phone about a year ago, after they were getting plagued with cold callers.

A Google search will find their website and I bought my unit via Amazon for about £100.

The man who invented this won funding for it on Dragon’s Den, but it is still not widely advertised.

You can program the unit to block specified numbers, withheld numbers, international, business and mobile numbers, etc. You can choose how to block, eg with messages or allowing the call to connect but playing a “number unobtainable” tone.

But you can let through all known genuine numbers or apply a two-stage filtering process to new numbers of which you might be suspicious.

This unit has stopped all unwanted calls bothering my parents. I should add that I write this only as a totally satisfied customer; I have no other link to the company.

Mike Stevens, Churchlands, North Bradley.