Some AT&T Inc. customers will pay more when the company soon adjusts older wireless plan prices, which have not been increased in three years, according to an AT&T spokesperson.

The company is encouraging customers to upgrade to newer plans that offer additional features and, in some instances, a lower monthly cost, the company wrote in an email to S&P Global Market Intelligence.

This announcement comes after AT&T executives discussed the need to raise customer prices to adjust for inflation during its April 21 earnings call. AT&T CEO John Stankey indicated that the company may raise plan prices to help offset higher expenses, including growing labor costs.

"Broadly across the board in the economy right now, we are seeing inflationary pressures ... I don't see the wireless industry being immune from that," Stankey said.

AT&T's average revenue per postpaid mobility user slipped 0.5% year over year in the first quarter, to $48.88. Looking at phone-only average revenue per user, growth was down 0.2%, which executives attributed primarily to promotional discount pricing. Average revenue per user for T-Mobile US Inc. and average revenue per account at Verizon Communications Inc. were both up slightly in first-quarter year-over-year comparisons, but inflationary pressures could soon be reflected more clearly in wireless metrics.

AT&T and its wireless competitors all have room to upsell customers, according to data from a recent MediaConsensus survey from Kagan, a media research group within S&P Global Market Intelligence. About two-thirds of AT&T subscribers surveyed by Kagan were already on unlimited data plans. Meanwhile, just 13% of T-Mobile customers were on the carrier's most expensive Magenta MAX plan. Verizon has the most room to upsell — just 10% of its subscriber lines are on the carrier's most expensive pricing plan.