Callback

Callback is designed to make it easy to get in contact with those who are less able to answer the phone themselves.

Install Callback on the device of the person/phone you wish to talk with, setup a trigger phrase and enable the service.

Then, when you need to talk, send the target phone an SMS containing the trigger phrase and wait for it to automatically call you back. (Starting on speakerphone if the option is set)

Useful as:
A means to get in contact with someone who is otherwise unable to call you themselves
A simple baby monitor