Why do I call it a Smart Doorbell?
A classic doorbell can be defined as a signalling device typically placed near an entry door to a building. When a visitor presses a button the bell rings inside the building, alerting the occupant to the presence of the visitor.
The smart doorbell, works in that manner however when a visitor presses the doorbell button it notifies the occupant with an images of the visitor and also sends an email, sms and push notification (to an smart device) to the occupant in case they are not able to hear the bell ringing.
If the button is pressed a pre-recorded voice notification is played for the occupant to check the door as there might be someone, this happens concurrently with an sms notification while a picture of the visitor is captured and sent to both email and push notification. Initially there was a feature to enable 2-way real-time video communication but due to network latency and high resource usage the feature was deprecated.
Comment if you need the code
See code snippets(Initial Revision): https://mmphego.wordpress.com/2015/01/11/smart-doorbell-using-rpi-with-voice-and-email-notification/