I've built https://cophone.io - your online smartphone, complete with a phone number. It is an Android system running in the cloud that you can access via a browser, even on mobiles. Things like microphone and webcam work nicely, so you can even have meetings on cophone. After iterating on it for a while, customers seem very happy and now growing day by day. No marketing so far, just being out there and posting on various channels once in a while.