You can forward e-mails, voice and fax messages. Since only numbers can be entered on a telephone, you can only forward messages to "telephone numbers" and "fax numbers".
The options vary depending on the message type, i.e. you can forward a voice message only as voice and an e-mail and a fax message only as fax.
If you forward a voice message, you can add a comment.
This is of course not possible when forwarding as fax.