T-Mobile US Inc. is luring AT&T Inc.'s unlimited grandfathered customers with a new deal.

Starting June 20, AT&T customers on grandfathered data unlimited for Apple Inc.'s iPhone on 4G LTE with visual voicemail plan can switch to T-Mobile, keep their number and get T-Mobile's WTH (Why the Hike) Deal — a $250 one-time offer, the company said June 14.

T-Mobile also is offering up to $650 via trade-in credit and a prepaid card to cover the cost of switching when a user signs up for T-Mobile ONE.

With T-Mobile ONE, users get unlimited LTE data, a Netflix Inc. subscription included on family plans, free texting all flight long and a free hour of smartphone Wi-Fi on Gogo Inc.-equipped domestic flights while traveling, plus free data roaming and texting in more than 140 countries and destinations.