Lower the speaking volume dial or listening volume dial on the base. For most telephones the correct setting is position 3.
If your desk phone has a volume control, lower until the distortion disappears.
If the distortion is still present, adjust the headset volume control to lower the headset speaker volume. If the distortion persists, lower the listening volume dial on the base.
Make sure there is at least 12 inches between your base and your computer, and 6 inches between your base and your telephone.