** Shares of telecommunications services firm EchoStar SATS.O fall 9% to $22

** The U.S. Federal Communications Commission has told SATS' chairman Charlie Ergen it would investigate the company's compliance with federal requirements to build a nationwide 5G network, WSJ reports

** EchoStar owns the Dish Network pay-TV brand and Boost Mobile's wireless service

** SATS has spent years wiring thousands of cellphone towers to help Boost become a wireless operator, but the project has been slow-going, report says

** SATS has risen 5.6% YTD, as of last close

(Reporting by Juby Babu in Mexico City)

((Juby.Babu@thomsonreuters.com;))