After the Calling Search Space of the Directory Number Configuration is changed, the Call Forward to voicemail stops to work. Subsequently, the calls to this directory number rings out and does not call foward on Ring No Answer.
This problem is tracked by the Cisco bug ID CSCsh13982. It is observed in the Cisco CallManager 5.0(4) 2000-1.
A possible workaround for this issue is on the Directory Number Configuration page of the phone. Enter a value for the No Answer Ring Duration. Save and choose Update. Then, reset the Directory Number.
In order to resolve this issue, install the ES 5.0(4) 2131-1. Open a case with Cisco Technical Support with the TAC Service Request Tool in order to obtain an appropriate Enterprise Special patch.