The No. 2 U.S. mobile service provider said it had 151,000 net new subscribers in the quarter, compared with the average expectation for 358,000, according to five analysts contacted by Reuters.
Its bigger rival, Verizon Wireless, added 1.5 million subscribers in the quarter.
AT&T said a shortage of the latest iPhone, which went on sale in the last week of the quarter, meant that the vast majority of third-quarter iPhone sales went to existing customers, stunting its growth of new customers.
Slow customer growth likely helped the company post better-than-expected earnings for the quarter as new customers come with a hefty cost for wireless service providers.
AT&T’s profit rose to $3.64 billion, or 63 cents per share, from $3.62 billion, or 61 cents per share, and was 3 cents ahead of Wall Street expectations, according to Thomson Reuters I/B/E/S.
However, revenue fell to $31.46 billion from $31.48 billion and missed the analysts’ average estimate of $31.59 billion, according to Thomson Reuters I/B/E/S.
The company also raised its target for 2012 free cash flow by $2 billion to $18 billion.
Reporting by Sinead Carew; Editing by Lisa Von Ahn and Jeffrey Benkoe