Sprint Nextel says it’s going to start phasing out the Nextel part of its network in 2013, a decision that follows near-constant subscriber losses since Sprint bought Nextel in 2005.

Sprint Nextel Corp. has about 10.6 million subscribers on the Nextel network, some of whom are using it under the Boost Mobile brand. Sprint said Monday it will offer them Nextel’s signature push-to-talk function on the Sprint network instead.

The phase-out by the Overland Park, Kan.-based company is part of a network modernization plan that will cost $4 billion to $5 billion. It is aimed at saving Sprint $10 billion to $11 billion over seven years. It’s hiring Alcatel-Lucent, Ericsson and Samsung for the upgrade.