* Core earnings rise 7% to 778 million euros

* Cellnex changed strategy in 2022 to offload assets, reduce debt

* Won credit rating upgrade from S&P last month, a key goal

BARCELONA, April 25 (Reuters) - Spain's Cellnex on Thursday posted a 7% rise in first-quarter adjusted earnings before interest, taxes, depreciation and amortisation to 778 million euros ($831 million) as revenue grew 7% thanks to the expansion of its towers' network.

Europe's largest mobile phone tower operator still booked a first-quarter net loss of 39 million euros but more than halved it from a year earlier.

Cellnex, which expanded through debt to become one of Europe's largest telecom companies, changed strategy in 2022 amid sharply rising interest rates and began disposing assets to reduce its net debt, which totalled 17.3 billion euros in the first quarter of this year, slightly lower than in 2023.

S&P upgraded last month its credit rating for Cellnex to investment grade, which had been a main goal this year for the Spanish company.

Cellnex sold its Irish business in March. It also said on Thursday it expects to receive in May non-binding offers for its Austrian business, adding that its eventual sale could accelerate a potential share repurchase programme.

Saudi Arabia's STC Group and sovereign fund PIF may bid for Cellnex's Austrian unit, El Economista newspaper reported on Wednesday, citing unidentified industry sources.

($1 = 0.9360 euros) (Reporting by Joan Faus, editing by Inti Landauro and Elaine Hardcastle)