When your phone plays the message, "Device Not Registered", or has no dial tone, you should visually look at the gateway device. It is a small black (and maybe grey) device with Grandstream on the face of it. When it is fully functional and working, there will be green or blue LED's illuminated. First check to ensure the Internet is active via a desktop or laptop computer. Do not just open a browser; do a page refresh or go to a new site to ensure the Internet is up.
Check your network cable to ensure it is plugged into the Grandstream device and router properly - pull both ends out of the respective devices and re-insert them being sure to hear the click of the locking device.
If the Internet is OK, the first step to correct this problem is to power cycle the Grandstream phone adapter. Give it about 3 minutes to complete. If successful, the Device Not Registered message will not play back, and you will be able to make a call.
A 'power cycle' is unplugging the device's power cord and waiting a few seconds (5-10) before plugging it back in.
Never press the reset button on the device without instructions from your service provider. If you do, there is a good chance you will have to bring the device back to your service provider to get it re-programmed (charges may apply).
The Device Not Registered message is still playing after rebooting the Grandstream device, you may try power cycling the modem/router combo or modem and router.
If both of these procedures fail to correct the problem, please call your service provider or open a ticket by sending an email to firstname.lastname@example.org describing your problem and what you have done so far. Someone will respond promptly.