BRUSSELS (Reuters) - Belgian mobile phone operator Mobistar grew its customer base for the first time in two years and slightly increased its outlook for the full year. The company gained 19,900 postpaid and prepaid customers in the third quarter, a reversal of a downward trend which started in the third quarter of 2012. At that time a new law came into force in Belgium which limited the maximum duration of customer contracts to six months, increasing competition and the number of users switching operator, so called churn, across the sector. The group had lost 52,700 customers in the second quarter of 2014, excluding machine to machine customers and those who buy access to Mobistar's network through a third party operator such as cable group Telenet. Core profit fell 22 percent in the third quarter to 70.7 million euros (55.99 million pounds), just above the 69.2 million expected in a Reuters poll of four analysts. Mobistar slightly raised the bottom end of its outlook range and now expects 2014 core profit to be between 260 and 280 million euros, up from 250 to 280 million euros. This equates to a 20 to 28 percent drop from 2013. The group, which had to pull the plug on a fixed line offer in 2013 as it was not successful, said it would restart such services by the end of the year. The Belgian regulator forced cable operators, such as Telenet and Numericable, to give wholesale access to third parties in an effort to increase competition for broadband internet and digital TV. (Reporting by Robert-Jan Bartunek; Editing by Anand Basu)