It is not possible to stop the Call Forward All (CFA) option on IP Phones that are configured with it in these Cisco CallManager versions:
All the calls that come to those phones go straight to voice mail. Even after those IP Phones are taken completely out of the system, calls to those phones still go to voice mail.
This issue is documented in Cisco bug ID CSCsj09236.
One possible workaround for this issue is to clear the CFA from the CCMAdmin page of the Cisco CallManager where the IP Phones are registered.
This issue is fixed in these versions of Cisco CallManager:
Open a case with Cisco Technical Support with the TAC Service Request Tool in order to obtain an appropriate Engineering Special patch.
This document applies to CallManager 4.x