Verizon Communications Inc. is raising prices on some existing wireless plans to help boost revenue and offset slumping subscriber growth.

Starting with September bills, customers on Mix and Match plans will pay $3 more for single lines and $5 extra for multiple lines, Chief Financial Officer Tony Skiadas said Wednesday during at an investor conference.

The price hike is Verizon’s third increase in two years. Last month the carrier marked up its wireless home internet prices by $10. Rival AT&T Inc. this month began charging $2.50 more per month for its top-tier wireless plan.

Verizon and AT&T, two of the three largest US wireless carriers, have been raising prices on more popular offerings in an attempt to widen profit margins as mobile phone subscriber growth slows. Both carriers have been signing up fewer customers than T-Mobile US Inc. and are losing market share to cable companies Comcast Corp and Charter Communications Inc., which have been offering free mobile lines.