The reason for this request is that certain VoIP adapters, notably the Sipura, have their own set of feature codes that conflict with the codes used in FreePBX. For example, Sipura uses the following just for Call Waiting:
CW_Act_Code Enable Call Waiting on all calls *56
CW_Deact_Code Disable Call Waiting on all calls *57
CW_Per_Call_Act_Code Enable Call Waiting for the next call *71
CW_Per_Call_Deact_Code Disable Call Waiting for the next call *70
Whereas FreePBX by default turns Call Waiting off for all extensions and (again by default) uses *70 to activate Call Waiting and *71 to deactivate. So, note that dialing *70 will never get passed to Asterisk, the Sipura intercepts it and thinks that you want to deactivate call waiting for the next call, which may in fact be the opposite of what you intend!
One way around that would be to turn Call Waiting on at Asterisk and let the adapter have the final say on whether CW is on or off but at present there's no way to do that other than dialing *70 (which doesn't work for the reasons described above) or changing the feature codes associated with Call Waiting for ALL users. What I'd prefer to see is, for each extension, a way to see and if necessary, change which features are activated/deactivated from within FreePBX, so that the administrator could make changes in feature activation if the user is incapable of doing so, or cannot figure out how to do so.