(Adds details on mobile phone, TV business)
BRUSSELS, April 28 (Reuters) - Belgian cable operator Telenet Group Holding NV posted a better-than-expected core profit in the first quarter as an end to promotions with free or discounted phones pushed costs down significantly.
Core profit rose 18 percent in the first quarter to 237.8 million euros ($329 million), above the 216 million euros expected in a Reuters poll of five analysts.
Telenet said its operating costs fell by almost a quarter compared with the same period last year, including a one-off gain of 12.5 million euros. Even without this item, Telenet's profit would have beaten expectations.
Telenet, which uses the network of Belgium's Mobistar to offer its mobile services, said the number of mobile customers grew by a quarter from the same period last year to just under 780,000.
Telenet also saw strong growth in its new on-demand television service "Rex & Rio", which saw a 55 percent increase in customers from the last quarter of 2013, following a promotional campaign.
Despite the strong result in the first quarter, Telenet kept its outlook for core profit (EBITDA) to grow by 5 to 6 percent over the course of the year, with revenues rising 6 to 7 percent.
Telenet, which is majority owned by U.S. group Liberty Global, offers its services in the north of Belgium and in parts of Brussels. ($1 = 0.7227 Euros) (Reporting by Robert-Jan Bartunek; Editing by Anand Basu and Chris Gallagher)