LONDON — The text message from a young boy, written in broken English on a no-frills cellphone, was frightening enough to set off a frantic, trans-Atlantic search that saved the lives of 15 migrants trapped in a locked truck in England. The message flashed on the cellphone of volunteer Liz Clegg, who was attending a conference in New York: “I ned halp darivar no stap car no oksijan in the car no signal iam in the cantenar.