I have been using Voip on my vigor 2800Vg for several years without any hitches however I have had my system scanned for vunerabilities and the following recommendations made:
Threat:

An attacker can gather information about existing users. When a user sends an INVITE message to the SIP server, it usually verifies whether the client exists and sends back a response. If the server is a proxy server and does not have any sort of session management or authentication enabled, an attacker can send an INVITE request and determine whether the user exists based on the response.
Impact:

Solution

Enable proxy authorization at the SIP proxy server, enable authentication at the SIP register server, and configure the SIP client to not respond if it receives the INVITE message from a different URL.

My problem is I cannot find controls on the 2800Vg to correct these issues
I have search on the web and can see these controls on lesser routers, but not on mine..