If the trunk is up and configured and on the Dashboard its showing as BLUE, it means that the grandstream has resolved the DNS and authenticated correctly which means incoming calls will work and your Hostname, username, and pasword are all correct.
the issue is most likely incorrect Sip string on outbound calls
and i’d be guessing the From domain is not what the Provider is expecting.
so in your Edit SIP trunk try the following.
try changing the “from domain” to match the “host name”
if that doesn’t work
try changing the “from domain” to your local WAN IP you can get the Wan IP from http://ipchicken.com
failing that you need to call the Provider and ask why the call is being rejected
they’ll tell you that the format need to be “xyz@abc”
you can alter the setting to match their expected format
xyz = “Username”
everything after the @ = “From Domain”
Hope this helps