Heads up, AT&T customers, because your monthly bill is increasing. 

AT&T has quietly increased an "administrative fee" that it applies to most of AT&T's postpaid phone lines. Up until about a year ago, that fee was $0.76 per month, but it's since grown a couple times, and this month it was raised to $1.99 per month. BTIG Research analyst Walter Piecyk notes that this fee should not apply to prepaid customers.

When asked why this fee is being increased, AT&T said that it helps to cover costs for things like cell site maintenance. "This is a standard administrative fee across the wireless industry, which helps cover costs we incur for items like cell site maintenance and interconnection between carriers," the carrier told CNBC.

With this latest administrative fee increase, it's estimated that AT&T could get around $800 million in additional annual revenue. 

While this increase may not sound like much at first, it's worth noting that this fee also applies to each postpaid line on an account. If you've got multiple lines, $1.99 for each adds up quickly. The increase is likely even more frustrating to AT&T customers because it's not going to improve their plan by giving them more mobile hotspot or extra international usage or something like that. 

AT&T customers, have you spotted this increased administrative fee on your monthly bill?