You mentioned 'different location and cluster' , is this a call across an ICT?
Just to confirm, does any other user in same location as user A experience the same issue? If not, then to troubleshoot this issue we should do the following:
Compare the device config of the working and non working user on location A. If it is the same then collect detailed callmanager traces for a test call ( preferably for working and non working scenario ) from both the clusters.
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-communications-manager-callmanager/10124-cm-trace.html
This will allow to check the difference in signaling in the two scenarios.
HTH
Manish