I set up an HT-486, a few months ago, and could not get it to work at all until I down loaded the latest firmware.
Go to the Grandstream firmware update page and set up the download parameters as listed :
Grandstream - Innovative IP Telephony
Also, get a copy of the matching User's Manual from the support pages.
Try some test numbers as Martin suggests, and if this still does not work, I'll try to post the essentials of the configuration, when I get some more time.