Walgreen Co said on Friday it will increase staffing at its call centers and offer discounts as it tries to hang on to customers once it stops filling prescriptions for Express Scripts Inc members on Sunday.

Walgreen has been battling with the pharmacy benefits manager (PBM) since June 21, when Walgreen said the companies could not agree on terms for a new contract. The current contract expires on Saturday.

The top drugstore chain, which operates 7,810 locations, is offering discounts next month on an annual membership fee and giving coupons to some Express Scripts members to keep them from defecting to rivals such as CVS Caremark Corp and Rite Aid Corp .

The retailer said that about 120 Express Scripts health plans, employers and other clients have switched PBMs so members could maintain access to Walgreen's stores. Last week it had said that number was about 100.

The increased staffing at the company's call centers is aimed at helping Express Scripts members find a way to still be able to fill prescriptions at Walgreen.

(Reporting By Phil Wahba; Editing by Gerald E. McCormick)