Instead of replying via text, you can physically go back to Saul at the Nomad camp and tell him about Panam's plan. Rewards: A free Mizutani Shion "Coyote" (red variant). A large sum of Eddies (€$27,000).

If you are referring to the quest "With a Little Help From My Friends," Saul Bright will text you asking for Panam’s location.

The text typically arrives shortly after you reach the locomotive control tower.