Recently acquired 2004 C240 with 30K miles. What a great car!
However, all to often, an audible tone sounds. With the radio or CD on, the volume mutes, the double tone sounds, volume comes back. It sound as if a telephone ring, but I do not utilize the telephone functions. When I press the answer or hang up button (can't remember which) when the tone sounds, the tone stops but radio volume stays muted.

Any help would be appreciated.

 

a clip of the sound would help or a you tube video. not sure what sound your talking about but the tape cassette door being open can cause a double beep sound..if you have a tape deck. usually if a benz is making some sort of beeping or audible noise it will notify you in the cluster of the reason for the sound. kind of like the E-brake sound when leaving the e brake on.
but you said it mutes the radio sound? maybe teleaid/ecall is activating and cuts out radio, but you will normally get a notification on the cluster or radio screen of an Ecall.

 

Thanks very much. I'll capture video and post.