How can I start a Hangouts phone call with Applescript/Terminal?
This script I wrote works for me.  You may need to adjust a few of the delay values, depending on the speed in which your computer launches your browser etc.

I actually wrote this script and saved it as an application and then I enabled it as a dictation command

tell application "Safari"
    activate
    set URL of document 1 to "https://hangouts.google.com/"
    delay 8 --adjust this value if needed
    set the clipboard to "1-234-567-8910" -- enter the real phone number
    tell application "System Events"
        click button 5 of UI element 1 of scroll area 1 of group 1 of group 1 of tab group 1 of splitter group 1 of window "Google Hangouts" of application process "Safari"
        delay 1
        click button "New conversation" of UI element 1 of scroll area 1 of group 11 of UI element 1 of scroll area 1 of group 1 of group 1 of tab group 1 of splitter group 1 of window "Google Hangouts" of application process "Safari"
        keystroke (the clipboard)
        delay 2
        key code 36
    end tell
end tell