If I desire only to make outgoing telephone calls from my Android, which must I download?
Google Voice gives you a second phone number linked to your original. You can use it to call outbound from the app. Google voice can also recieve calls as well as send and receive texts.
^^My Google Hangouts menu^^
Google Dialer (If I have the correct product) seems to be just a phone management app, just like the typical "Phone" app that comes preinstalled on most phones. It gives you a dialer screen and a Google Duos video call screen as well as spam protection and caller ID. However, it doesn't look like it comes with a phone number, it uses the one from your SIM. So it'll make outbound calls, but only if you have a SIM card installed.
EDIT: Hangouts Dialer (Not Google Phone, sorry, got that wrong the first time) is a dialer for Google Hangouts (hence the name). It's a way of making phone calls without using minutes. You can call any phone number, and it shows on the receiving end the verified number connected with your device. I believe it can also be used in combination with Google Voice, using that number as your verified number.