Hi md nazmulfrombrooklyn,

I'm sorry to hear you are having this issue with your iPhone. I also apologize, I'm a bit unclear exactly what you are describing or in what context you are seeing this message. If you are trying to activate your iPhone during setup and are getting a "Device Unknown" error, you may find the troubleshooting steps outlined in the following article helpful, up to and including contacting Apple Support directly if necessary:

If you can't activate your iPhone - Apple Support

Regards