In fact, in a clustered environment, every phone can register to any ICP in the cluster regardless of whether the phone resides there as primary, secondary, or not at all. All cluster elements know where all phones reside (except Local Only DNs) so any controller they contact will tell them which ICP is their primary and secondary. We have a 13 site cluster setup this way, and all phones register to Node 1 via DHCP, from there it tells them their resilient pair to use.
The only time this becomes a problems is if Node 1 is done more than a day or so... DHCP leases of phones start expiring and they can't renew if the node is down, this has only happened once in 10 years and that was because of a storm on a Friday night took out the system and no one really discovered it until 7am Monday.