A T-Mobile retail store in downtown Portland, Oregon. (Photo by Matthew Keys for The Desk)

T-Mobile is taking a page out of the “Big Wireless” playbook by implementing price adjustments on some older plans that will see most long-time customers pay more money for the same service.

Affected customers began receiving text messages this week warning them of the price increase, which could see them pay between $2 and $5 extra on top of their old monthly rate.

The rate adjustment appears to primarily impact older wireless tiers like T-Mobile One and T-Mobile Simple Choice, according to KnowTechie, which published a copy of a text message received by a long-time customer.

A T-Mobile executive affirmed the price increases in a memo circulated to employees on Wednesday, noting it was the “first time in nearly a decade” that the wireless provider had adjusted its price on long-time customers. It comes several months after...