*#67# (Check Call Forwarding)
If you want to know whether your calls are forwarded to another number, use this code. That'll tell you whether call forwarding is active on your device, and it'll also show you the number your calls are being forwarded to. In case calls are being forwarded when your number's busy or when you reject a call, you'll know about that too.
In most cases, you'll only see this as your carrier's official voicemail service. And you also have the option to change this setting by going into your Android's call settings.