The phone stops reporting. A dot on a map would just freeze. Nightleg said so plainly and started estimating from his pace.
"Probably at Aid Station 5." The crew logged him in at 11:21. Every later ETA tightened around that real time.
He slowed, and Nightleg slowed with him. It projects from the most recent stretch run, not the morning's average, so the times the crew drove to stayed believable.