Shift DC signal to AC
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Decoupling capacitor for DTMF signal.

You need to decouple the DTMF signal with a series capacitor. It will block the DC component from reaching the DTMF decoder.

Unfortunately you haven't given a circuit schematic of your setup or any component values so I can't be any more specific.