Verizon Communications Inc (NYSE:VZ) shares dropped 7% after warning that "soft" subscriber growth was likely in the first quarter.
Rival carriers were attracting customers with promotions, the US wireless group said at an investor conference.
Verizon dialed down on promotions after heavy spending in the December quarter, but rivals did not, said chief revenue officer Frank Boulben at a media, internet & telecom conference held by Deutsche Bank in New York.
A "slow start" in phone upgrades had also been seen in the first quarter, he said, a sign that customers are holding back on purchases amid growing worries about the US economy.