When your customers are on hold, they’re essentially a captive audience. They’re already waiting to speak with you, so why not take advantage of that downtime and create a sales pitch that sells more of your goods and services while they wait?
Here is our top 5 recommendation of what to look for in a quality on hold message provider:

source: I have samsung galaxy y . i want to play music during call and callee can listen music. is there any app in android market? mikemangada Level 10 (Genius) Unfortunately there is no feature like this, If someone call you all media will be closed because how would you able to hear the person talking if the music is playing.
For those still using analogue or digital phone systems, voice on ISDN circuits has a bandwidth of around 4000Hz. The audio from an analogue phone is sampled at 8000Hz, and because of the nyquist theorem, must be low-pass filtered to 4000Hz. So any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are good at recognising human speech, so this is considered "toll-quality" voice, i.e. good enough to pay for. This is the reason why certain vocalised letters are difficult or impossible to tell apart on the phone. For example, the differences between B, P, T, C, etc. are almost entirely above 4000Hz.