By Alina Selyukh WASHINGTON (Reuters) – Major U.S. wireless carriers on Thursday pledged to make it easier for consumers to unlock their mobile phones for use on competitors networks, responding to pressure from consumer groups and the top U.S. communications regulator. Verizon Wireless, ATT Inc, Sprint Corp, T-Mobile US and U.S. Cellular agreed to clearly notify customers when their devices are eligible for unlocking and to process unlocking requests within two business days, said wireless industry group CTIA. U.S. wireless carriers often lock smartphones to their networks as a way to encourage consumers to renew their mobile contracts.