Europe's biggest 4G mobile phone operator EE reported a 1.1% fall in operating revenue in the first quarter of 2015.
EE, the biggest mobile phone operator in the UK, posted revenues of £1,469m (€2,052m, $2,224m) in the first three month of the year.
The company is set to be taken over by the British telecoms group BT, in a move that would create a communications giant across broadband, mobile, TV and fixed-line phones.
The £12.5bn deal still requires regulatory approval, after it was challenged by rival telecoms firms. If approved, BT would buy all Orange and Deutsche Telekom's EE shares.
EE has sought to expand its customer base, reaching a total of 31million customers, including 9.3 million 4G customers.
The company has said it is "firmly on track to reach our target of 14 million 4G customers by year-end".
If the impact of regulatory changes were discounted from the company's balance sheets, its operating revenues would have increased by 0.3% in the first quarter compared to the previous year.