Call forwarding can be setup at the line level from CM Administrator by an administrator. User can also setup call forwarding at the line level from user web pages or an end user can forward there phone by pressing the CFwdALL softkey on their phone.
In your situation there are a number of things that could be going on. So many that it would not be possible to address all of them in this format. If this is not a consistent problem then it is most likely not a configuration issue. The majority of the time it comes down to user error.