Third line's the charm.

T-Mobile customers that have two or more lines of service can get an additional line for free. Usually, customers can buy in T-Mobile One for $70 a month for the first line, $20 a month for a second line, and $20 a month for each additional line.

Any T-Mobile subscriber with One or Simple Choice is eligible for the extra line, starting on March 1. T-Mobile is currently offering an Un-carrier deal that offers two lines for $100, and three lines for $140. The third free line could save customers up to $40 a month.

"Today, I'm thanking customers by giving them one of the things they want the most -- a way to connect more of their family or more of their devices all the time," T-Mobile president John Legere said in a statement.

The extra line isn't limited to just phones, and can be used on smartwatches and tablets, too.

The offer comes as T-Mobile continues its uphill battle against Verizon. The two telecommunications giants are competing on every front, including who really has the most reliable network in the land. RootMetrics declared Verizon the winner for the seventh test in a row, while T-Mobile stands by OpenSignal's report that it's the fastest network.

T-Mobile's push to add an extra line is its latest "thanks" to current customers, sticking to the company's theme of smothering customers with free stuff. It launched T-Mobile Tuesdays in 2016, dishing out freebies like pizza and movie tickets to people using its app.

Here's a link to T-Mobile's full press release.