We need a bit more information in order to help you properly. Are you struggling to perform *any* phonon calculation in CASTEP? If so, the links Ariadne sent should help, and of course there are many other resources on castep.org.
If instead the problem is specific to CaMnO3, then we need to know what the problem is. Does CASTEP abort? What does the error file say? Does it run, but you doubt the results? What are the results, and why do you think they are strange?