If 3300 then two options
1: They may be set up in call rerouting: usually as a call rerouting first alternative will be to voicemail. You just need to change that to (normally) 1 so it no longer works. You can check to see if 1 is the normal one by looking in call rerouting first alternative; looking for everything set to normal. No.1 is usually left alone for this purpose.
2: There could be individual call forwarding set on the sets. Use the superkey to go to call forwarding and see if anything is programmed /set. if so then just turn it off. (use the review key to check).