Whilst heavily implied it is a mission given by Trevor Philips, it is not offered via a phone message but started by walking into the blue corona (marked by a star icon on the player's radar) at the top of the road opposite Hookies in Grand Theft Auto Online Freemode. The mission is available from 02:00 to 10:00.
This is one of six co-op "contact" missions not directly given by the contacts in GTA Online (including Coveted, Potshot, Crystal Clear Out II and III, and Coasting). It can be accessed immediately upon reaching the required Rank by the Start>Online>Jobs menu (PlayStation 3 and Xbox 360 only) or by walking into the job corona on the map (although the map coronas are limited by time of day).
The team must kill the couriers, and deliver the meth to Trevor's Trailer in Sandy Shores. Once that is done, the mission is complete. If playing this in a team of two, both couriers will drop packets of meth and both must be delivered. If playing the mission alone, only one meth package is required to be delivered.