About 15 per cent of Etisalat's mobile customers are "inactive", according to an analysis of figures issued by the regulator.
Mobile subscribers are described as inactive when they do not use their line for three months or more.
Unlike its rival du, the UAE's first mobile operator does not report active subscribers, which makes it difficult to compare its performance against its competitor.
Etisalat claimed to have 7,764,000 mobile customers at the end of last year. But the number of subscribers that are using their lines could have been as low as 6,593,000. By comparison, du reported 4,333,000 active subscribers at the end of last year, based on customers using their phone lines in the previous three months.
While Etisalat does not publicly disclose its active subscriber base, the Telecommunications Regulatory Authority said there were 10,926,000 active subscribers at the end of last year. Martin Mabbutt, a telecoms analyst at Nomura, said Etisalat's reporting practices were "unhelpful". "Etisalat's numbers are usually much greater than the regulator's numbers," he said.
"It tends not to be awfully helpful in terms of making comparisons. Etisalat reports numbers that are not in line with what the regulator reports." Mr Mabbutt said it would benefit analysts if Etisalat was to change its reporting practice.
"It's in everyone's interest in the long term for people to have numbers people can work with," he said.
Etisalat did not respond to a request for comment.
Jawad Abbassi, the founder and general manager of the Arab Advisors Group, a regional consultancy, said the two companies had "two different ways of reporting" their mobile subscriber bases.
"du calculates it based on the three-month activity rule," he said. "Etisalat reports it as active … regardless of whether [a subscriber] made a phone call or not."
Mr Abbassi said reporting active mobile subscriptions could lead to subscriber numbers of between 15 and 20 per cent lower than the reporting method used by Etisalat.
"The three-month method is more conservative in the context of the UAE," said Mr Abbassi. "Usually we'd say that the regulator of any market would compare apples to apples."
However, he said the subscriber numbers were not as important a measure compared with other aspects of the telecoms operators' financials. "So long as both operators report revenues, who cares about lines?" said Mr Abbassi.