The ESP8266 has an i2s bus that can do duplex audio in the right configs, needs external chip.
- https://github.com/cnlohr/esp8266duplexi2s/tree/master/user
- https://github.com/espressif/ESP8266_MP3_DECODER
Looks like using ./configure-legacy as a base is reasonable to start, there is some support for platforms like RTEMS, which implies it will be portable further, http://www.pjsip.org/using.htm .