Somehow Izuku knew where he was - instinctively he understood his mission, and how dangerous it potentially was. "Sorry, let me borrow this." He snatched one of the bystanders' phones. They didn't understand what was happening, but he thought he did. Izuku did remember Endeavor's phone number, but ten years back, he wasn't sure if the other had the same one. He called twice, no response, and time was of the essence. So he frantically texted your son is dying, then tossed the phone back to the stranger he took it from before diving into the depth of Sekoto Peak, searching for the source of the fire.