Authentication Errors like the 401 or 403 mean that the server is rejecting your connection, usually for Registration, but sometimes for calling.
Typically these errors are caused by incorrect Account settings, usually username and password. Check these again with your VoIP provider and be sure the account is correctly configured.
Sometimes a server controls where a user may connect from - i.e. the local network, but not the Internet. If you can connect in some places but get the Authentication Error in another, this may be the case. Contact your Server Administrator about this.
Sometimes these messages are accompanied by additional text explaining the error - if so, ask your VoIP provider or Server Administrator about these errors.