Brazil's telecommunications regulator said cellular companies Oi, Claro and TIM Participacoes will be barred from selling new mobile plans in certain states until they present a plan to improve service quality.
In a press conference, Anatel said TIM Participacoes, a unit of Italy's Telecom Italia, must halt sales in 19 states.
Oi, Brazil's largest telecommunications provider, and Claro, a unit of Mexican magnate Carlos Slim's America Movil, must stop selling new plans in five states and three states, respectively.
The firms, which collectively have a 70 percent share of Brazil's wireless market, will have 30 days to present a plan to improve service or face further sanctions.
Those plans must map out a series of actions designed to improve network quality and call integrity, reduce service interruption and provide better customer service.
Anatel said it made the decision after receiving an increased number of customer complaints over the past year and carrying out inspections.
Anatel chief Joao Rezende termed the measure "extreme" but said that "the increase in the number of customers must go hand-in-hand with quality of service improvement."
Non-compliance with the measure announced Wednesday will result in a daily fine of 200,000 reais (about $99,911).
Wireless market leader Vivo, controlled by Spain's Telefonica, is not barred from selling in any states although that firm also must present a plan to improve service quality, the regulator said.
Oi released a statement criticizing the action, saying Anatel's analysis is "outdated" and does not take into account the company's investments over the past 12 months to improve network quality.
The Rio de Janeiro-based company said it will invest 6 billion reais (nearly $3 billion) this year in Brazil, or 1 billion reais ($495 million) more than in 2011. EFE