From my experience, the build-in software AEC does not always work well. For example when the echo tail exceeds some minimum threshold or there is some volatility in the echo path, the build-in software AEC breaks. If you find out that this is the case in your system, then there are two options:
- Upgrade to a card with AEC.
- Use an external software AEC, like PBXMate, that will do the echo cancellation on behalf of your devices.