When you use Partner ACS to forward calls, you are connecting two phone lines together inside the ACS. If they are regular phone lines, the quality will be poor - low volume. (It works better with T1 lines.)
But if you have noise or breaking-up sounds, that doesn't sound like normal. You should only have low volume.
You can experiment with this by performing a conference call between 1 inside party and 2 outside parties. You will be able to hear both outside parties clearly, and they will each hear you clearly. But the outside parties will have some trouble hearing each other. Yet there should be no static or break-up sounds.
As pointed out, alternative schemes, such as having your telco forward the call for you, work better. As does using T1 digital circuits. With either of these approaches, you bypass the analog lines.