MetroPCS Communications could not attract enough new paying customers to offset the loss of millions of subscribers fleeing its mobile phone service.
The company, which released its subscriber data for the quarter ending December 31, 2012, on Monday, said that additions to its service fell 29 percent.
The number of total subscribers fell 5 percent to 8.8 million, the first time that Metro PCS lost customers during a typically strong quarter, noted UBS analyst John Hodulik.
Shares of the company were down 2.6 percent at $9.48 on Monday morning.
The rate at which customers dropped the service, or "churn," was 3.6 percent for the quarter, almost unchanged from the same quarter a year ago.
The quarterly subscriber results come as MetroPCS awaits approval to merge with Deutsche Telekom unit T-Mobile USA.
"We believe this underscores the urgency with which the company pursued merger partners before reaching a deal with T-Mobile in October," Hodulik wrote.
On a more positive note, the Dallas-based company is gaining subscribers to its next generation network - 4G LTE or long term evolution. It said it increased the number of LTE customers 117 percent to more than 2.2 million, quarter-on-quarter.
(Reporting by Jennifer Saba in New York; editing by Matthew Lewis)