The Advanced Audio Distribution Profile (A2DP) is a source/sink configuration. In an normal scenario, you have a source (for example, and Android phone) and a sink (Bluetooth headset/speakers). But in your case, the Android phone would have to act as a sink to the dumbphone source. This is not currently supported, even though the phone supports the A2DP profile.
One solution would be to install a custom Bluetooth stack, which defines which profiles are supported on the device. This would additionally require a software component that can bypass the Android Framework and use the stack directly, since the framework lacks A2DP sink support. So far, no such modifications can be found.