I think this project is particularly interesting as there doesn't seem to be much development regarding calling on the Raspberry Pi. I believe Skype might be a good API to use because calling is free and it has already been around for awhile, so there is quite a bit of documentation avaliable.

I picked the Raspberry Pi 3 because it has the most computing capacity and has multiple USB ports for connecting other devices.

There will be options to control and schedule when and where the Hello! device turns on just to ensure that you won't be suddenly woken up at 3am by a sound from the other paired Hello! device.

Current Task

Identifying suitable speakers to add to the Raspberry Pi 3. I am also currently writing code to do basic calling via Skype API.

List of items needed

1. Raspberry Pi 3
2. Skype account
3. Microphone and speaker
4. PIR sensor
5. LED lights for feedback
6. Buttons/touch interface for option selection

Schedule

1. Identify suitable speakers and microphone and add them to the Raspberry Pi. Verify that they can work and can be turned off and on in software.
2. Write code that will allow Skype clients to call each other
3. Add a PIR sensor and use it as a trigger to start the call sequence. Add a button to end the call sequence
4. Add touch capabilities to the device, use it as a trigger to start and stop the call sequence
5. Add LEDs for visual feedback. 
6. Start designing a case to house the Hello! Device
7. Add pairing sequence so that any two devices can be paired together