If you have a Windows phone, there's a chance that you may be using the app Truecaller, which offers caller ID for folks that aren't on your contact list. According to some tips given to the folks at WMPU, the app has been taken off of the Windows 10 store, despite sticking around for users on Windows 8.1.
Thankfully, Truecaller isn't going away. According to a response to a Truecaller user on the app's Twitter page, Truecaller is soon going to be available again for Windows 10 users.
@sharathknot Don't worry! It will be available soon for Win10!
— Truecaller help (@TruecallerHelp) September 1, 2015