Cellphones are small, so I can hold one in my right hand and have my right thumb in a position to hit every key, leaving my left hand free. Finger-poking takes two hands–one hand to hold the phone, and one hand to dial.
Conversely, I can’t imagine why anyone would take up two hands to dial a cellphone when they’ve got a perfectly good thumb. 
I normally use my own finger.
Thumb for cell. I think index for home phone, but I’ve never really paid attention.
Twenty.
I was surprised first by the question and then surprised by my answer. My cell phone, I hold in my right hand and push the buttons with my right thumb. My cordless is too big and heavy to hold one-handed and so I hold it in my left hand and push with the right index finger. And dial phones (I still have a couple around the house), I dial with my right index finger. It was only nine years ago when I broke down and paid the !@#$% phone company for touch-tone service. The bastards charged $2.50 a month for that, but when I finally got high speed internet.
72
Exactly. It’s so convenient to do it one handed, if you have a small phone (even my portable home phone is small enough to be used that way), and makes more sense to me.
As for phones with actual dials, we had one of those as late as the 80s. I don’t think I ever got a chance to use it though, since we upgraded to a modern version by the time I was able to call people unassisted.