Steve Smith, one of the inventors of the product, appeared on Dragons’ Den – a show where new businesses pitch for investment from five ‘dragons’ – asking for £100,000 in return for a 7.5% stake in the firm.

The Truecall device works by using two lists of phone numbers, ‘star’ and ‘zap’. Friends and relatives are put on the star list and their calls are put straight through but numbers on the zap list receive a message (either automated or recorded by the user) informing them that their call is not wanted. Callers not on either list are asked to identify themselves to the machine before the recipient decides whether they wish to take the call.

Smith’s fellow inventor, John Price, told Research in an interview last year that the duo developed the product to block calls that are exempt from the Telephone Preference Service, which includes calls from research agencies, after seeing the effects of unwanted calls on friends and relatives.

All five dragons offered Smith cash in return for varying stakes in the firm, but the inventor made a counter-offer to Peter Jones – who made his fortune in the telecoms industry – and the two eventually negotiated a deal for 12.5% of the company .

Click here to see Smith’s presentation to the dragons.