OK thanks for that, I will try it, however the issue is quite weird.
It *looks* like a connectivity issue - it looks like the failed over handset can't see the primary controller to fail back to.
Additionally, this handset on the resilient controller cannot dial handsets on the primary *if those extensions were recently dialled*.
If I dial from the failed over handset to a handset on the primary that I haven't previously dialled, it works fine.
This happens to a small number of handsets whenever a group of handsets on a site loses its connection to the primary. Most come back fine. Some need a reboot.
Ultimately, what this looks like is a networking issue on the handset. The handset may be holding onto routes that are no longer valid. When the secondary controller gives the dialling handset the IP address of the dialled handset, the handset can't get to handsets it already knows the IP for (from earlier calls).
I would have thought it would just use the default route and so either all succeed or all fail. I need to see the routing table on the handset. I hope that's possible, will investigate further.