Call forwarding is a function of your carrier, not of your phone. Conditional call forwarding can be forward-when-busy, forward-when-offline, etc. Call your voicemail and check the menu to see if there are settings. If not, call your carrier for help in finding out what conditions your forwarding is set for and/or change the settings.
If you do a google search you will quickly see this is a samsung related problem in my opinion. may well be your carrier settings that are wrong but it has something to do with samsung why this message appears. all the posts i have read online about this are samsung phones. I only searched the problem because someone at work with a samsung phone was having the same issue. another samsung fail.