On the Kilimanjaro, there is reception all the way to the summit! However, the signal is not very strong and can be hard to find. Sometimes you’d need to look for a good spot.
Pay attention: There are no or limited possibilities to charge your phone. So make sure your phone is fully charged before the big climb starts and reduce the battery usage.
Advice: Check the costs for mobile data use in Tanzania before departure. You could choose to switch your phone off or to put it on airplane mode to prevent high costs for mobile data.