Executives with three U.S. mobile carriers have voiced support for an effort in the U.S. Congress to encourage mobile carriers to offer services to customers that would render smartphones and tablets inoperable if they're stolen. Executives with Verizon Wireless, T-Mobile USA and C Spire Wireless all said Wednesday they're taking steps to offer kill switches on mobile devices they sell to consumers, although they stopped short of endorsing a bill that would require mobile carriers to offer kill switch services.