[Fire HD 8 and HD 10 General] DragonFire1024:
Update, April 5, 2019: you can now make calls with Google voice.

1. Open the app, tap the phone icon then the dialer on the bottom right.

2. If it doesn't work, go into settings by tapping lines at top left. Make sure you are set to receive calls, disconnect hangouts if you haven't.

3. Make sure you set your preference to use Wi-Fi to make calls.

4. Restart app, go back to step one.

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****OLD METHOD****

I'm making this post because this worked surprisingly well. Call clarity is near perfect and there's little to no delay whatsoever. To be honest, I wasn't expecting this to work well on this tablet let alone any other ones.

Google recently revamped Google Voice and Hangouts which all but combined the two apps into one. If you already have a Google account, you can easily sign up to use Google voice here. To use this feature you'll need to do so. Once you've done so, you're literally a few steps away from receiving and making phone calls. Also install Activity Launcher if you haven't already done so.

1) It's not required, but it is helpful to download and install Google Contacts.

2) Download and install Google Voice (once you've gotten a Voice number). You can now send SMS messages to anyone with a regular phone number and you can also receive texts from any phone number. You can also now setup voicemail which will then allow anyone to call your Voice number and leave you a voice message. In the next steps, you'll be able to answer those phone calls and even make them!

3) Download and install Google Hangouts and Hangouts Dialer. No additional sign-up is needed. You should be automatically signed in when opening Hangouts. In Hangouts tap the three bars at the top left and select settings and tap your account if signed in.

4) Scroll to the Google Voice section and select the option to 'receive incoming phone calls.' Otherwise incoming calls will be directed to the voicemail you just setup in the Voice app. Voice will transcribe the message for you, if able to do so and you can listen to and read them right from the Voice app. In the next option, if you so desire, you can select to receive messages in Hangouts that are sent to your Voice number.

5) Using Activity Launcher, tap top left, all activities, scroll to 'Media Storage' and select the 'Hangouts Call' as your ringtone (It should currently be set to none). If you select the 'default ringtone' no sound will be played so you have to select the Hangouts tone. (If you figure out how to add more ringtones, please post a reply with the steps to do so. I've not gotten that far or even know if you can do so).

6) Now have someone call your new number. You should be able to answer the call on your tablet. Now open hangouts and select the dialer. You can now call any number you want, for free, provided you're on a good WiFi network.

You can setup the apps up to your liking, but you won't be able to get calls through the Voice app anymore. In older versions, Voice had it's own dialer, or used the one built into your phone/device if there was one. In the Voice app you can also decide if you want voicemail messages forwarded to your Gmail.

Note: If you have a working telephone number already, you can link it through Voice. I currently don't have cellular service and haven't for some time so you'll have to figure out that part on your own :P