UPDATE 2-Brazil lifts ban on new mobile plan sales

* Anatel to monitor investments to ensure quality

* Networks, customer service shoddy because of rapid growth

* Carriers commit to invest additional $1.96 bln through 2014

BRASILIA, Aug 2 (Reuters) - Brazil lifted a partial ban on new sales of mobile phone plans on Thursday, but warned it would remain vigilant to ensure operators fulfill investments to improve overburdened networks and poor customer service.

Lifting the ban, telecoms regulator Anatel said that the affected operators, Brazil's three leading mobile carriers, had provided satisfactory plans to ensure better service and coverage.

The decision benefits TIM Participações, a unit of Telecom Italia, Brazilian carrier Oi and Claro, a unit of Mexico's America Movil. Sales of new plans can resume on Friday, Anatel said.

On July 18, Anatel ordered the carriers to stop selling new mobile plans in states where they were providing poor customer service and spotty wireless coverage.

The move, a surprisingly harsh punishment in a country where regulatory agencies have long been lax, was widely applauded by consumers increasingly frustrated with poor cell phone service.

Jammed networks, and slow and unresponsive customer service centers, are the result of years of non-stop growth for mobile carriers in Brazil. The saturation has increasingly been compared to the country's rickety road, rail, and air infrastructure, which the government hopes to improve before Brazil hosts the World Cup of soccer in 2014 and Olympics in 2016.

During the country's recent economic boom, millions of new customers bought phones and cell plans. In São Paulo, South America's biggest city, regulators recently had to add a ninth digit to local cell phones because eight-number combinations were running out.

Despite the growth, regulators say carriers have failed to reinvest in their services. TIM, Brazil's No. 2 wireless carrier, was banned from selling plans in 19 states, Oi in five states and Claro in three.

Anatel lifted the suspension after the carriers committed to increase investments over the next few years by a combined 4 billion reais (US$1.96 billion), bringing total spending plans by the companies in the period to 20 billion reais ($9.8 billion).

However, Anatel also said that it would start examining the investments on a quarterly basis to ensure service and coverage are improving, and that the carriers that do not comply will face fines and penalties.

"This improvement plan is just a first step. We'll continue to closely monitor the improvements in services and the networks," João Rezende, president of Anatel, said at a news conference in the capital, Brasilia.

"We recognize that services aren't going to improve overnight, but in the call centers, it's possible to resolve the problems in the short term," he said, adding that network improvements would take longer, probably four to six months.

The Anatel ban caught the telecoms companies by surprise. The carriers first lashed out at the measure as extreme, but then scrambled to come up with investment plans to resume business as soon as possible.

In a statement, Claro said it had explained to Anatel how it would "maintain the quality of its network, improve its customer service, and offer the capacity necessary to meet demand during the World Cup and Olympics."

Oi, in its own statement, "reiterates its commitment to improved quality of mobile services."

TIM shares rose 3.57 percent on Thursday in anticipation of the announcement. Oi shares ended 0.64 percent lower, falling in line with the benchmark Bovespa index in Sao Paulo.