By Ollie Stone-Lee
BBC News political reporter
The watchdog who polices the system designed to prevent nuisance calls has dismissed claims he is not doing enough to stop firms flouting the rules.
Silent calls are an irritation for many householders
Information commissioner Richard Thomas deals with complaints from homes listed on the Telephone Preference Service, which bans unsolicited marketing calls.
He has yet to punish any firm, despite about 29,000 complaints last year.
But Mr Thomas says he may take action against one firm in what could be the first prosecution of its kind.
Companies which fail to comply with the watchdog's enforcement notices face fines of up to £5,000.
Showing his teeth?
More than 11 million telephone numbers are listed on the TPS system, designed to prevent "cold calling" from sales and marketing firms.
Figures given in parliamentary answers suggest there were more than 29,000 complaints last year, either to TPS itself or Mr Thomas, about abuse of the TPS rules and other privacy and electronic communication laws.
Mr Thomas sent out more than 1,000 warning letters but has yet to fine any company.
Liberal Democrat trade spokesman Norman Lamb has said it is "incredible" nobody has faced punitive action.
And the Daily Mail has said it is time the commissioner "showed his teeth".
In an interview with the BBC News website, Mr Thomas brushed off the press criticism, saying: "You get flak from time to time."
"Telephone Preference Service is working incredibly well," he said.
"Not enough people know they can register their details with the Telephone Preference Service but the numbers have gone up quite dramatically in recent years.
Automated calls featuring Sir Sean Connery sparked complaints
"If your name is on there then there is a dramatic reduction - almost elimination - of marketing calls."
He acknowledged there were "occasional complaints" but the vast majority of those were either "oversights" or a small company which did not quite understand how the system worked.
A phone call or letter from his office meant they would not break the rules again, he said.
Mr Thomas explained his approach was to be "selective to be effective".
"We can't do everything," he argued. "We are only going to take action where people are either causing widespread harm, are ignoring our warnings, are causing a lot of harm to a particular individual."
He vowed to "go for" any firm which was persistently the subject of complaints - but that was not the case at the moment.
And he added: "There is one company at the moment where we have had a couple of warnings to them and we suspect they may still be going on with it.
"That is one we may be taking action against if we have the evidence. That would be highly exceptional."
After last year's general election, Mr Thomas issued an enforcement order against the Scottish National Party over automated messages featuring an appeal from movie star Sir Sean Connery.
The party is appealing against the decision at the Information Tribunal in March saying the rules do not apply to political marketing.
Mr Thomas said most political parties and commercial companies realised the public were "very resistant" to being disturbed by telephone calls and using them could be counter-productive.
Lib Dem spokesman Mr Lamb welcomed news that Mr Thomas was looking at using his existing powers to clamp down on some companies.
"The current system has been allowed to get out of hand and people up and down the country will warmly welcome this action," he said.