Note that your mobile will not have signal and your GPS tracker may not record you as moving in the areas with red dots as signal is weak or non existent. Note that this route is from last year and will change.
From Grächen to Zermatt, mobiles work almost everywhere except in a few places where there is no line of sight to the valley bottom. There is no network close to checkpoint 2, although climbing up 50m above the checkpoint can find you a weak signal.
After crossing the pass after CP5 (the first checkpoint in Italy), the signal can disappear for short periods. 
Stage 3, there is poor coverage in the valleys before and after CP9, Alagna, and non-existent coverage between Passo Turlo and CP10 Quarazza. 
Stage4
Their is mainly signal up until Monte Moro, but at the checkpoint the signal is weak as you move away from the edge looking down to Macugnaga valley. After Monte Moro, there is no signal until the end of the Mattmark dam in Switzerland.
After crossing the Monte Moro pass, there is no network signal until the end of the Mattmark dam.
Between CP13 Saas Fee and CP14 Hannigalp, the signal is week in the valleys that cut into the mountain.