I've encountered the same issue. I went step by step but no call out.
Are you using the LAN or WAN on your HT-813?
Use Wireshark to see what udp.port == 5060 is providing.
When using the LAN port on the HT813 the device would not accept the SIP Request: INVITE , when I configured the HT813 to use the WAN port it provided a Status: 100 Trying,Status: 180 Ringing and Status: 200 OK (Invite) and called out.
Make sure you allow on the grandstream:
WAN side HTTP/Telnet access: YES
when using the WAN on the HT813 or you wont be able to configure further on the WAN port.