T-Mobile promises a new $15 wireless plan if its $26.5 billion Sprint deal goes through.
T-Mobile is also promising free internet to emergency first responders for 10 years and to low-income households with children for five years. The $15 plan is for anyone, but comes with just 2 gigabytes of data per month.
Federal regulators have approved the deal.
But attorneys general of 15 states and the District of Columbia aim to block it, saying it will raise prices for consumers. A trial is to start in December.
T-Mobile has already made promises to get the deal done, including providing coverage in rural areas and not raising prices for three years.
Regulators could fine T-Mobile for breaking the earlier promises, but T-Mobile isn't legally required to fulfill the new ones.