When hiking the Inca Trail, you won’t be fully reachable as you might be used to. On the first day, you may have signal only in the morning at certain points along the Trail. On the second day, there is no signal at all.
On the third day, you might get a signal at specific points (Puyupatamarka and Wiñay Wayna). On the fourth day, during the final part of the hike and at Machu Picchu, you can use your phone again. The guide carries special communication equipment for emergencies.