When connected to some VOIP service providers you may notice a distortion in the voice quality when making phone calls. This often is due to the Codec used in the router phone settings. The default codec used is G.729a/b.
If you have a low speed Internet connection, try changing the codec to G.723. On a higher speed connection you can choose G.711A. This codec requires uses 64Kbps bandwidth compared to 6.4Kbps for G723.