Apple’s stock continued to slide on Tuesday morning as some analysts suggest AT&T’s earnings point to a possible rare miss for the Cupertino, California-based company. Wall Street expects Apple to report iPhone channel sales of 32 million units when it posts its earnings after the market closes on Tuesday, but several analysts have made last-minute adjustments to their estimates following AT&T’s earnings report on Tuesday morning.

Apple’s top iPhone carrier partner reported that it activated 4.3 million iPhone handsets last quarter, up from 3.6 million in the first quarter last year but missing many estimates, and Verizon reported last week that it activated 3.2 million iPhones. According to several firms, these activation figures point to a potential Apple miss.

“Apple’s stock has corrected just over 10% from its all-time high last week,” Jeffries & Company analysts wrote Tuesday morning in a note to investors. “We believe this has been partially due to concerns over carrier subsidies eroding and penetration rates losing steam. We disagree on both counts and provide a brief look at subsidies as well as the iPhone and iPad opportunity. AT&T’s and Verizon’s results imply cuts are required to our CQ2 Apple estimates.”

The firm continued, “AT&T activated 4.3M iPhones in Q1 (-43% Q/Q) vs. our expectation of 6M. Last week Verizon reported activating 3.2M (-24% Q/Q) vs. our expectation of 4M. iPhone share fell slightly at both (from 81% to 78% at AT&T; from 55% to 51% at Verizon). We believe this bodes poorly for Apple’s CQ2 guidance. Instead of 32M iPhone shipments in CQ2 we now believe 28M-30M is more likely (we currently estimate 32M).”

BTIG Research analyst Walter Piecyk points to a slower-than-expected upgrade rate as the culprit for a potential iPhone sales miss. “The lower than expected number of iPhone activations appears to be driven by a lower than expected upgrade rate of 7% versus our estimate of 9% and Q4 upgrade rate of 11.9% resulting in total devices sold in calendar Q1 of 7.1 million versus our estimate of 8.7 million, a 17% decline from last year,” the analyst wrote in a note on Tuesday. “This is a significant slowdown and likely driven by the company’s new upgrade policies.”

Piecyk believes that new upgrade policies could have an even greater impact on iPhone sales in future quarters. While he thinks iPhone sales in China are likely to offset slowed sales in the U.S. to an extent, Piecyk reiterated his iPhone sales estimates of 33 million units this past quarter and 27.5 million in the third fiscal quarter.

Updated to correct BTIG’s iPhone sales estimates