Why not use an existing service with IFTTT? Since you're talking about a DigiSpark connected to a computer anyways, the computer could communicate, for example, through simple files, via Dropbox or Google Drive.
Let's say you have a sensor at home, and you want to receive notifications about it on your phone while you're out.
Your computer monitors the sensor (maybe looking for the temperature to go above a certain point, or detecting the opening of a door switch), then writes the status to a file in your Dropbox folder. You could have an IFTTT recipe that watches that folder, and sends you an SMS.
Going in the other direction, you could have a GMail account setup, and an IFTTT recipe that creates files in Dropbox upon receiving a new email. Your program could monitor the folder for new files, read them in, parse/validate them, and take actions based on them (activate a light or buzzer, trip a relay, etc.).