Nigerian regulators have threatened to fine and stop three telecommunications companies from signing up new cellular phone subscribers over complaints about poor service in a nation reliant on mobile calls, officials said on Thursday.

Bharti Airtel Ltd of India, local firm Globacom Ltd and South Africa-based MTN Group Ltd all face a deadline at the end of November to improve their service, said Reuben Muoka, a spokesman for the Nigerian Communications Commission.

Otherwise, they will be liable for fines and halted from expanding their customer base, Muoka said.

The firms will be judged on users being able to make and complete calls, as well as the quality of the calls, Muoka said. Mobile phone users in Nigeria often juggle several phones with different carriers to make sure they can make calls due to the poor conditions.

Muoka said the commission already has equipment in place to monitor performance by the companies.

"It's not really a warning," Muoka told The Associated Press. "It is an ultimatum because within the specified period, the law comes into force and stands."

MTN gave public notice of the deadline in its third quarter results released today. Long the dominant provider in Nigeria, the company said it now has 41.1 million subscribers in the nation after 10 years of doing business there.

"MTN is engaged with the regulator on the content and consequences of the allegations," MTN said.

Globacom, known as Glo, is the second largest mobile phone service provider in Nigeria, with more than 25 million subscribers. A company spokesman could not be reached for comment today.

Airtel entered the Nigerian market last year after purchasing Kuwait-based Zain. Emeka Opara, a spokesman in Nigeria for Airtel, declined to comment.

Nigeria, long troubled by pothole-littered roads and little electricity, has relied on mobile phones since the government granted the public access to them about a decade ago. Landlines remain almost nonexistent, as the state-run telephone company has collapsed and repeated efforts to sell it to a private company have failed.

The ultimatum by the commission comes as Nigeria, home to 150 million people, continues its explosive growth, making it a lucrative market for mobile phone service providers. The arrival of Airtel sparked a price war in the market, with local phone calls now down to pennies a minute.