Basically, I think you are correct, there are none. I would suggest additionally that DRBD over a VPN is just begging for a catastrophic Split-Brain failure (if not worse)
Just Musing . . .
Historically , HA for Asterisk was basically a homebrewed admixture of abstracting the "state" of an asterisk instance and placing that "state" onto a Master/Slave DRBD filesystem and a "heartbeat" recipe to notice the absense of the "other server" and take the step of firing up local mysql,Apache and network services as appropriate. This I know because five or seven years ago I participated in some of those recipes.
As far as I know the FreePBX recipe likely suborned such a recipe, obfuscated it and then made it commercial, we will never know though.
Things have changed, virtualization is now virtually ubiquitous, no longer do you need to limit the Filesystem to Master/Slave (and the inherent "split-brain" problem) And the whole ,shebang can be on the self same FileSystem (think about the advantages . . .)
GlusterFS is intrinsically Master/Master, Corosync betters Heartbeat, So a simple KVM+corosync recipe is free and "better", for an easy GUI look at ProxMox, it does it all for you ;-) .Two machines, a networked filesystem, a self scripted failover for the machine and it's IP, no double licensing of software, (the Asterisk instance is the same, it just moves to another host.) Would that not work for most?
For XIVO , it uses realtime database and Asterisk has for a longtime had res_corosync for MWI and extension states if you have the two machines able to multicast to each other, perhaps that could be added? and then you could have both instances active for even quicker failover. Just Corosync/BGP to do the routing.
(Perhaps better to have a real proxy between the Asteri and the Internet IMHO though)
JM2CWAE but comments welcomed/anticipated
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