The Publisher has just finished the upgrade procedure, and the Publisher is currently running Cisco CallManager version 3.1(4b).
Cisco CallManager was designed specifically for this type of scenario. Find one of the IP phones, and go to Settings > Network Configuration. Scroll down to the CallManager settings. Usually CallManager 1 is the Subscriber, and CallManager 2 is the Publisher. However, there are differences in this setup. If CallManager 2 is the Publisher, then the phones are set correctly to re-home to the Publisher.
Stop Cisco CallManager services on the Subscriber. The phones then re-home. Re-homing is nearly instant. Users notice the screen refresh, and any calls in progress are protected.
For example, in an environment with Cisco CallManager and Cisco Unity, an upgrade from CallManager 3.1(4a) to 3.2(3) may also require an upgrade to the Cisco Unity-CallManager Telephony Application Programming Interface (TAPI) service provider (TSP). The latest, and only available, software version of this TSP is 7.0. To download this software, refer to Cisco Downloads.