I also had the problem on my Samsung S7...but it was only for numbers of people who subscribe to "WhatsApp". (It appears as a listing on the contact detail page under the "View More".) I uninstalled my "WhatsApp" app and the +1 dialing listing was GONE. Re-installed WhatsApp and the +1 calling did NOT reappear. Hope this helps.
Sorry about the WhatsApp solution, it was wrong. While it did eliminate the +1 from the contact listing, it still added it back when dialing. Solution from T Mobile tech for my Verizon Samsung S7 phone:
Tap Phone Icon
Menu (three dots, upper right corner)
Scroll down and tap on "More Settings"
Scroll down to "Assisted Settings"
Tap and next to "Manual", turn it off (swipe to the left)