The G.729 codec cannot be used with IVM, nor any other NCH software. The reason being, it is a proprietary codec meaning that there is a license fee to use it and NCH has decide to only use free/open codecs for the software.
You don't need G.729 to fix your quality issues though, the only reason your VoIP provider recommended it is because it provides very good quality and requires relatively low bandwidth.
First you must try and define what you mean by poor audio quality. With VoIP there are 2 types of poor audio connections and they have completely different solutions, so it is important to figure out what the cause of the problem is.
Quality Type 1: Audio clarity. This is where the audio does not break up, stutter, or scramble, it is simply difficult to understand what the other person is saying and sounds like a they are holding a towel over the mouth piece.
-> Solution: Change to a higher bandwidth codec (or select the option in IVM to use 'higher bandwidth'). Higher bandwidth codecs typically offer much better audio clarity for clear, crisp sound however they do take up more bandwidth on your Internet connection and if that bandwidth is not available, you might have problem 2 (below).
Quality Type 2: Connection issues. This is where the audio randomly starts breaking up, skipping/missing words, stutters, or scrambles up. This is the result of using a codec that needs too much bandwidth, more bandwidth than is available on your connection. As a result, the audio packets can become lost, discarded, ignored (leaving blank/silent stutter spots in speech or missed words) or the audio packets can arrive at the receiving end in a mixed up order, resulting in scrambled speech.
-> Solution: Change to a lower bandwidth codec (or select the option in IVM to use 'lower bandwidth'). Lower bandwidth codecs, as the name suggest use much less bandwidth (e.g. 10kbps vs 80kbps) and are more suitable for slower connections, partially blocked connections, or congested (i.e. heavily used) connections.
Note that you can also solve many audio quality issues by doing the following:
1) Set-up port forwarding on your router and forward UDP ports 8,000 through 16,000 and TCP port 5060 through 5061 to your IVM computer.
2) If your router has the feature, enable QoS. QoS (or Quality of Service) can be used to give VoIP packets priority when they travel through your network. This ensures the audio packets are sent out to the Internet first and in the right order rather than having the router treat them like everything else which can lead to them being sent in the wrong order, or having random pauses while some web data is sent and the VoIP packets are put in queue to be sent later.
3) Run the built-in network set-up wizard in IVM