Please check the Call Manager Group configuration under System menu. It should have both the CMs listed. And this Call Manager Group should be listed under the Device Pool to which the IP Phones belong. In that case, when the primary(should be the subscriber) CM goes down, the phones will directly rehome to the secondary (should be the publisher which has the Tftp server also) without restarting.
In your case, the phones are not rehoming to the secondary means please check if the CallManager Group is correctly configured. The phone neednot contact the Tftp server again when the primary CM becomes unavailable, the phone already has this information who its secondary CM is, in it, when the phone first downloaded its configuration file from tftp server upon power-up or reset.
Also, you donot need a redundant tftp server, cuz the phones or other devices contact the tftp server only upon power-up or reset, and not when a CM in a redundant group goes down.
