[Freeswitch-users] Strange problem with ringback (distorted sound)

Hi all, I have freeswitch configured with standard media mode (no proxy no bypass). When a call arrives (through external profile), FS sets ringback and sends 183 session progress. Unfortunately the calling party (i.e. a cellular phone), hears a distorted ring sound. With proxy mode, of course the calling party hears a correct ring sound. What could be the reason? Best regards, Carlo Dimaggio