CUCM cluster setup model is clustering over WAN. We have site A having Pub,Sub1,Sub2,TFTP1 and Site B having Sub3,Sub4,TFTP 2.
Assuming we setup two TFTP server in option 150 on DHCP server and call manager group configured with redundancy of other side call processer and vice versa.
When site A phone lost connection assuming both with Sub1 and TFTP1 will phone able to regsiter to Sub3 or not.Will TFTP2 server information available at time when it loose connection to SiteA so that it can register with Sub3.
When lossing connecion to sub1 will phone try to register with Sub3 or will not be able to do so as the information from where it was knowing other sub (TFTP1) also get disconnected.
At the end of the day, the phone needs to be able to contact the TFTP server in the DHCP option 150 field. (Technically, if it has a known working config, that's stored in memory and it will fall back to that config if it can't contact the TFTP server after several attempts).
Once it pulls the config down from TFTP, that will have the list of CM servers as you have it specified in the CM group. The phone then needs to be able to contact one of these CM servers.
Note that if phone A registers to Sub1 and phone B registers to Sub3, if the WAN is down, they will not be able to call each other unless each site has a local gateway registered to the local Sub.