Phone users who sign up to the telemarketing opt-out Telephone Preference Service see their nuisance calls drop by just one-third, new figures show.
Ofcom said rogue companies who simply "ignore" the TPS register, or consumers who have unwittingly given their consent for telemarketing, meant registering with the service saw an average drop in unsolicited live sales calls of 31% per month.
The Information Commissioner's Office (ICO), which has responsibility for enforcing action against companies who ignore the TPS, called for "no further delay" in strengthening its powers.
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The TPS is a free service for consumers that supposedly enables them to opt-out of receiving unsolicited live sales or marketing calls.
However Ofcom also gave the example of consumers giving consent to marketing, sometimes without realising, by ticking a box on an online sales form, meaning they would continue to receive legitimate sales calls after signing up to the TPS.
A task force has been set up to look at the issue of marketing consent.
The study did find that those on the TPS register were much more likely to be completely free of unsolicited live marketing sales calls.
Nearly half (45%) of those registered with the TPS as part of the study did not receive any live sales calls, compared to a quarter (26%) of those who were not.
And registering with the TPS resulted in a 35% fall in the volume of all nuisance calls received per month.
Ofcom urged consumers to be careful when giving out contact details and to look carefully at any marketing 'opt-in' and 'opt-out' boxes.