T-Mobile US agreed to buy more spectrum licences in the 600MHz band, bolstering its nationwide 5G service by shoring up access previously granted under lease agreements.

In a Securities and Exchange (SEC) filing, the Deutsche Telekom-owned US operator stated it had agreed to pay a total of $3.5 billion for the licences to two Delaware-registered companies, Channel 51 License and LB License.

The airwaves cover 20 markets and licences range from 10MHz to 30MHz in each. T-Mobile noted it currently uses the licences through exclusive leasing agreements with the sellers.

T-Mobile previously spent $8 billion on 600MHz frequencies during a Federal Communication Commission (FCC) auction in April 2017.

The acquisition of Sprint means T-Mobile also owns or leases a large chunk of the mid-band 2.5GHz spectrum used in a higher-speed 5G service.

T-Mobile is currently taking part in the FCC’s Auction 108 for additional 2.5GHz licences.