LAGOS (Reuters) - Large billboards at Lagos airport urge travelers to call a hotline to report officials asking for bribes. But there is a problem with this attempt to fight the corruption that plagues Africa’s biggest economy.

A view showing the tower and departure sign outside the Murtala Mohammed International airport in Nigeria's commercial capital Lagos, May 11, 2017. REUTERS/Akintunde Akinleye

The phone number does not work, an indication of how little progress President Muhammadu Buhari has made in tackling a problem he promised to address when he was elected two years ago.

The government has fired customs and immigration officers accused of corruption, introduced staff rotation at passport and customs desks at Lagos airport to disrupt cozy networks, and set up the phone number to report demands for bribes.

That didn’t end a decades-long culture of corruption at the main gateway to Nigeria. But it did make officers more cautious as colleagues got fired, airport workers say.

But airlines and cargo firms say the fight against corruption has since faltered and government work has slowed down while Buhari has been on and off sick leave since January. On Sunday, the 74-year-old left again for treatment in London.

The government achieved results to begin with but officers have started acting with a sense of impunity again, said the country manager of an airline with daily flights to Nigeria, asking like others not to be named.

A presidency official acknowledged problems, such as the phone line being switched off for an unknown reason. He said a new number was planned as part of a package of new measures.

Executives say authorities have inadvertently made the problem worse by asking customs officers to be more aggressive in collecting duties to offset the slump in oil revenues that has pushed the West African nation into recession.

But the results are not always as intended.

“The way it works now is that a customs officer makes maximum demands but then says ‘If you don’t want to pay just give me something’,” said another one airline manager.

For Nigeria, corruption is a problem that could cost it 37 percent of its gross domestic product by 2030 if it is not tackled, PricewaterhouseCoopers said in a study last year.

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Built in the 1970s using Amsterdam’s Schiphol airport as a blueprint, Murtala Muhammad International Airport in Lagos has, like the rest of the country, seen little investment. There is no central air conditioning and some luggage carousels don’t work.

In most of the world’s airports officers only do random luggage checks, relying on dogs and x-ray machines to spot drugs or bombs.

But in Lagos officers from four state agencies search every bag, working from desks at check-in counters. To avoid searches delaying flights some airlines say they pay officers $25 for each flight. “If you don’t pay they will slow down searches.”

That doesn’t stop officers asking passengers for bribes, exploiting a vague ban on food items which can be resold. They have been known to confiscate a jar of jam unless money is paid.

In February, Vice President Yemi Osinbajo made a surprise visit to the airport, demanding to know why maintenance funds had not been used to fix air chillers and luggage carousels, officials said. He also raised the issue of bribes.

The next day the presidency sacked Nigeria’s civil aviation heads. At Lagos airport, officers were again more cautious for two weeks. But they went back to their old habits as there was no follow-up to the disciplinary blitz, airport workers said.

Two weeks later, Buhari returned from treatment in Britain, but he has not been active and the anti-corruption drive has been taken up by his deputy, Osinbajo.

A presidency official said Osinbajo had given orders to install cameras to identify bribe-takers. Three maintenance engineers had been removed and more staff are being reviewed.

“We have seen some slight improvements but it’s less than we expected,” said Bismarck Rewane, CEO of Lagos consultancy Financial Derivatives. “It’s a culture change that’s needed.”

That can be seen at the arrivals terminal door. It is off-limits to non-airport staff but people can still enter if they hand over a small bribes to policemen or have friends to get them in.

Baggage porters ask travelers for the equivalent of $10 to get through customs without searches, a Reuters correspondent at the airport said.

Authorities have also been unable to end the practise by some cargo import firms of paying customs officers a flat fee to avoid searches, executives said.

“It’s a payment shared by officers who then charge less or nothing for (import) duties,” said one executive. “It’s a win-win for officers getting cash and importers avoiding customs.”