IPLC is just a bandwidth pipe we get from a country to country. For Voice traffic we can use TDM, VoFR depending on the mux. for. eg. Alcatel 3600 supports TDM and VoFR as well.
I don’t think we learn the protocol by looking the LEDs, may be by lookign on the cards that we used in the MUX. say, in Alcatel FRE (Frame Relay Engine) card shows the usage of VoFr.
Hope this helps to some extent. Others views are also welcome.