@NathanG - I am guessing that the DSP circuits are used in making phone calls. According to the article these circuits convert analog signals to digital.
I think when you make a phone call that’s what happens. Your voice call (which is analog I believe) gets converted somehow to a digital signal, and this travels through a fiber optic network.
On the other side of the call I think the circuit works in reverse – or another device is used to convert the digital signal back to analog again. Analog to digital conversions are quite common nowadays in a lot of applications.