Hello,
Take a look at Dallas Semiconductor web site, which produces integrated circuits for this purpose. I was doing a digital telephony project a couple of years ago with PCM circuits(E1-30 channels because I live in Europe) and I have used microcontrollers together with some PCM circuits from Dallas. I forgot, nowdays you can find them under the Maxim brand. In any case the PCM integrated circuits synhronize themselves on the PCM stream and you can have an interrupt towards your MCU every time an data character inside the appropriate PCM channel is received and you can choose between 30 of them. You can configure the PCM demultiplexer in many ways because it has internal registers with an 8-bit bus as an control interface. You can also transmitt an byte which will be inserted in a PCM stream.