Upvote:3
Search for the nearest village/town that has taxi services. Phone them and tell them when and where you want to take a taxi. You will probably have to speak Italian (in Valle d'Aoste, you might be able to use French) and will probably have to negotiate a price. You cannot expect those taxi drivers to be bookable with a website or smartphone app.
If you are staying in a hotel, the hotel may be able to help you. If the nearest town or village has a tourist office, the tourist office may be able to help you. You may have to pay a premium if the nearest town is far away. If you are further off the beaten track, you're on your own.
From Bone to Usellières I would walk or hitch-hike.
It will be much easier to take a taxi /to/ the mountains than /from/ the mountains.