Unknown status bar icon (Samsung Galaxy S6, Android 5.1.1)
In short, it indicates that for the mobile calls the wifi will be used, if there's a good wifi signal.
I have found it there:

Small phone with Wi-Fi coming from it
This strange new symbol shows your phone’s set up for Wi-Fi calling. It’s a new feature brought in by the UK’s biggest networks to help give you the best call quality. If your phone doesn’t have good reception, like when you’re indoors, it’ll make phone calls using an available Wi-Fi network instead.