Shift DC signal to AC

Question

I want to make a DTMF decoder and use it on an internal DC phone line. I made a decoder with PIC microprocessor (Black DTMF decoder) that works with signals around 0V. My internal phone line consists of a 9V battery, a 330 ohm resister and a phone, all in a series circuit and the DTMF signal is read from resistor ends. My problem is that the signal is always above 0V (around 3.5V that changes if I use another phone) and I need to shift it around 0V but I don't know how?

Edited: This is my circuits diagram.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

^{simulate this circuit – 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.