T-Mobile USA has launched a new promotional offer for its family plan customers, allowing them to add up to three lines to their service agreement for free.

The Kids Are Free offer runs until Nov. 2 and applies to voice and messaging on up to three lines added to qualifying two-line family plans. The lines will be free until the end of 2011, after which customers will pay regular costs for them.

“The up-front cost of purchasing new phones and dealing with monthly service fees for an entire family can add up quickly, so this promotion is designed to make it easier for families,” said Ashley White, marketing director of value offers at T-Mobile USA, in a press release.

T-Mobile also launched a buy-on-get-one-free offer for its Samsung Vibrant and cut the prices on its new line of messaging phones to under $75 with rebate and two-year service agreement.

T-Mobile was not the only carrier to unveil a buy-one-get-one-free offer. Verizon Wireless is offering a buy-one-get-one-free deal on the new Samsung Fascinate, which launched online today. The device costs $199.99 after a $100 mail-in rebate with a new two-year contract.