Since you have DD-WRT, we have some more troubleshooting tools at our disposal.
If my guess was wrong and X_ProxyServerRedundancy does not solve the problem, I suggest you check /proc/net/ip_conntrack to see if a NAT connection is corrupted.
While the problem is happening, navigate to Administration >> Commands. Run the following commands one at a time, and post the results. If your public IP address appears, you can mask the last two octets with X to protect your privacy.
nvram show | grep ct_udp_timeout
grep obi.ip.address /proc/net/ip_conntrack
Obviously, replace obi.ip.address with its actual IP address.
That will give us the information we need to adjust your timeouts. However, I'm optimistic that X_ProxyServerRedundancy will work. This will cause the OBi to register to
sip.ca.anveo.com if a corrupted NAT connection prevents it from reaching
sip.anveo.com, and vice-versa.
Keep us posted.