* Q3 core profit down 8 pct vs 10 pct drop in Rtrs poll
* Keeps 2012 revenue, core profit outlook
* Cuts net profit outlook on billing system costs
* Shares up 5.5 pct (Adds analyst comments)
By Ethan Bilby
BRUSSELS, Oct 23 Belgian mobile phone operator Mobistar said it limited a fall in third-quarter profit by keeping a tight rein on costs and signing up more higher-value billed customers.
Mobistar, like other telecom operators, has suffered from an enforced reduction in roaming tariffs and mobile termination rates, the charge they levy to route competitors' calls, which should together cut revenue by some 60 million euros this year.
The company, majority-owned by France Telecom, confirmed its forecasts that 2012 turnover would fall by as much as 2 percent and that core profit would reach 460-500 million euros, down from 530.1 million euros in 2011.
Belgium's second-biggest mobile phone operator after Belgacom did, however, cut its guidance for net profit because of costs associated with upgrading its billing systems.
It said it expected a net profit this year of 157-182 million euros. It had previously forecast 170-195 million euros.
KBC Securities analyst Tom Simonts called the adjustment a "footnote" which did not reflect a worsening of the underlying performance.
Third-quarter core profit (EBITDA) fell by about 8 percent from last year to 119.7 million euros, not as steep a drop as the 10 percent decline on average predicted by a Reuters poll of 10 banks and brokers.
Mobistar shares rose 5.5 percent to 22.94 euros by 0935 GMT, although they are still down 46 percent in the year to date.
"The figures do clearly reflect a decline, but nothing of the sort that you could say it's not under control anymore," Simonts said.
Mobistar registered an increase in post-paid subscribers, which bring in more revenue for mobile operators, but a decrease in prepaid customers due to competition from more than 40 operators renting access to mobile networks.
However, the latter have also provided income for Mobistar, with some 700,000 people using its network via other operators from just over half a million a year earlier.
Mobistar said it had begun a new cost-savings plan which seeks to achieve at least 100 million euros additional savings between 2013 and 2015. They firm said the plan had already produced positive results during the third quarter.
Last year, Mobistar made a net profit of 221 million euros or 3.68 euros per share and paid out 3.70 euros in ordinary and extraordinary dividends.
Rival Telenet, which uses Mobistar's network for mobile calls, said last week that it had welcomed a record 65,000 customers to its mobile services after launching its new King and Kong combined data and telephone offerings in August.
Mobistar's own hopes to offer combined phone, television, and Internet are pinned on the deregulation of Belgium's cable sector, which is expected to happen in 2013. ($1 = 0.7651 euros) (Editing by Philip Blenkinsop and Helen Massy-Beresford)