VoIP beginner, looking for information

Without going into too much detail, I am hunting for information on building what is essentially a full-duplex intercom using VoIP. I am trying to find information on the detials of the UDP packet used, standard formats, how the Codec is used to compress the data, etc. Most of the data I can find is covering the higher level. I want low-level data, as I will be designing the circuit from the ground up. Any suggestions on where to find this information, or existing products would be helpfull.