Google Voice access is a free app that works on any Android device, phone or tablet. It works better with newer / higher spec devices but still works well on my 4 year old Nexus 5 running Android 5.0 .
Once downloaded and activated by saying, 'Ok Google, activate voice access' All the icons on the screen now have a specific number, you can open up apps by using their name e.g. Open Whatsapp or by saying their number. Once you have opened one screen the next screen then re-populates with more numbers and you can choose again, text is easily inputted also by voice.
There are a couple of example videos below showing how I send a WhatsApp message to my brother and also a slick video from Google showing it in action.