To best answer your question, let us briefly consider why compression is used in VoIP and what the nature of that compression is. The reason for that compression is to lower the size of voice packets to improve the effectiveness of transmission. This compression is made possible because of encoding schemes that can render audio signals more efficient. These encoding schemes can digitally process the packets within a VoIP voice transmission to emphasize the voice information (such as the words you are speaking), while suppressing background noise (such as unrelated conversation in a room or noise from machinery). VoIP encoding schemes can also direct the transmission to temporarily halt during pauses in a conversation -- lowering the bits unnecessarily carried during those short intervals.
VoIP uses various compression ratios, the lowest of which is 1:1, and the highest of which is around 12:1. As a general rule, the lower the compression ratio, the better the voice quality. The determining factor is available bandwidth, which is related to your VoIP setup and, of course, to the speed of the connection -- either Internet or proprietary -- that "carries" your VoIP session.
For more information, learn how many calls a T1 line can handle.