Thanks to u/Classic-Walk-1171, who replied to my post in r/freepbx, the issue was DNS.
The DHCP server was assigning it to a remote DNS, but the firewall is set to block all outbound traffic. Setting the DNS to point to the gateway fixed the issue. Don’t know why, but apparently it’s what the Grandstream device needed.
To answer other’s comments about the firewall, the ATA and phones are communicating to a FreePBX server all on the same VLAN. No firewall rules are needed. And yes, I could ping it (forgot to mention that earlier).
Anyway, even when it’s not DNS… it’s always DNS. :-)
SOLVED!
Thanks to u/Classic-Walk-1171, who replied to my post in r/freepbx, the issue was DNS.
The DHCP server was assigning it to a remote DNS, but the firewall is set to block all outbound traffic. Setting the DNS to point to the gateway fixed the issue. Don’t know why, but apparently it’s what the Grandstream device needed.
To answer other’s comments about the firewall, the ATA and phones are communicating to a FreePBX server all on the same VLAN. No firewall rules are needed. And yes, I could ping it (forgot to mention that earlier).
Anyway, even when it’s not DNS… it’s always DNS. :-)