Boost, a unit of Sprint Nextel , said on Thursday that it would extend its $50-a-month unlimited service plan to the entire Sprint network in an effort to improve customer growth and handset choice.

Boost has helped offset Sprint's customer losses since it launched its popular prepaid unlimited plan early last year on the company's separate iDen network, which Sprint took on through its purchase of Nextel Communications in 2005.

Boost is now looking to help improve customer numbers on Sprint's CDMA network by making its service plan available on that network on January 13. Sprint mostly uses the CDMA network to serve postpaid monthly bill paying customers.

Customers who sign onto Boost's CDMA service will have access to devices such as the BlackBerry Curve from Research In Motion , the first smartphone being offered under the Boost brand, the company said.

BlackBerry users will pay $60 a month instead of $50 to cover the service fees for RIM's mobile email service.

(Reporting by Sinead Carew; Editing by Lisa Von Ahn)