Now a days it is very hot topics about voip on symbian mobile. To Call Over the internet from Mobile phone helps peoples life easier.There are few commercial and free voip mobile dialer for symbian and android.

One of them is Fring.You can make voip sip calls from fring.You can also use MSN, yahoo and Google.It supports many devices such as Nokia N73,n71,n75,E65 etc. It also support Other platform like iphone. It supports g711,GSM codec only.

SIPDialer is a product of E-SOFT BILLING PTE LTD. They provide commercial service. Their SIPDialer supports Symbian s60 3rd ed and android phone. You can send a demo request  from their website www.e-softbilling.com. They provide 5 days demo to run own sip proxy. SIPDialer supports G729,AMR,GSM and g711 codec. SIPDialre has balance display, realtime call duration, incoming call, DTMF, phone book features.Proxy IP can be changed on the fly.

SIPDroid is another opensource application for android phone.you can make audio and video calls from SIPDroid.It is based on mjsip.It has gsm, g711,ilbc and speex codecs.

• To import a root certificate into the device, it must be in DER encoded format. To setup a certificate that is create using gentls_cert script use the following command

openssl x509 -in /usr/local/freeswitch/conf/ssl/CA/cacert.pem -out /usr/local/freeswitch/conf/ssl/CA/cacert.cer -outform DER

• Copy the new cacert.cer to the device, and run it in file manager. This will prompt you the save it as a trusted certificate. Do this and select all trust levels during installation.
• Setup your SIP Profile using “sips:” instead of “sip:” in Public Username, Proxy Server Address and Register Server Address, and set the appropriate port for your FreeSWITCH installation.

ATTENTION!!! Set the “Transport type” to “AUTO” or it will not work!

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