If you are using PJSIP 1.3

Edit config_site.h
#if defined(PJ_SYMBIAN) || PJ_SYMBIAN==1
#   define PJMEDIA_AUDIO_DEV_HAS_SYMB_APS       1
#   define PJMEDIA_AUDIO_DEV_HAS_SYMB_MDA       0
#endif

// …

#define PJMEDIA_CONF_USE_SWITCH_BOARD   1
// Below, enable all codecs you need
//…

//Enable passthrough codecs
#define PJMEDIA_HAS_PASSTHROUGH_CODECS  1

// Disable all passthrough codecs except PCMA and PCMU
#define PJMEDIA_HAS_PASSTHROUGH_CODEC_PCMU      1
#define PJMEDIA_HAS_PASSTHROUGH_CODEC_PCMA      1
#define PJMEDIA_HAS_PASSTHROUGH_CODEC_AMR       0
#define PJMEDIA_HAS_PASSTHROUGH_CODEC_G729      0
#define PJMEDIA_HAS_PASSTHROUGH_CODEC_ILBC      0

Edit you mmp project file:
#define SND_HAS_APS     1
#define SND_HAS_VAS     0
#define SND_HAS_MDA     0
Specify application UID in a global variable named APP_UID whose base type TPtrC, e.g:
TPtrC APP_UID = _L(“2000521C”);

Link the application to APS library, i.e:
LIBRARY         APSSession2.lib

Add MultimediaDD capability to your application, i.e. in application MMP:
CAPABILITY      MultimediaDD …
Install APS Server apsserver2.sisx to device, the installer comes with the APS SDK package i.e in folder InstallToDevice/AudioProxyServer243. You will need to do this only once for each device.

 
The complete step-by-step process is detailed in:
http://trac.pjsip.org/repos/wiki/APS
and
http://trac.pjsip.org/repos/wiki/Nokia_APS_VAS_Direct