Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving, from a computer system, a request comprising a phone number, identifying a primary channel and one or more secondary channels based on, at least in part, respective performance data of the primary and secondary channels, sending a first message comprising a first text string via the primary channel to a destination device associated with the phone number, after sending the first message, determining that a conversion event for the message and the primary channel did not occur within a specified time period, and based on the determining, sending a second message comprising the first text string via a particular secondary channel to the destination device.