Guys,

I found SIP calls quite working well on nokia n900 with the help of IPKall(Provides a completely free PSTN number if you have a sip account. PSTN only for incoming call) and PBXes(provides a free SIP account) and it works great for me(only incoming call), refer the link below for setting up the sip account in PBXes for incoming call,

How to setup PBXes, step by step

Note
1) First create a PBXes account as said in the above link and use that credentials to get a Washington DC number from IPKall.
2) When Setting up Trunk Provide 'voiper2.ipkall.com' instead of voiper/voiper2.ipkall.com.
3) Keep dummy credentials where ever IPKall information is asked for and you can fill once the IPKall account is created.
4) PBXes does not allow registration from Internet Explorer (atleast on my PC ), use Firefox instead.IE keeps giving the captcha error.

IPKall Account Details
(Link to IPKall Signup Page)

Settings on the phone

Address ---> <sip-extn>@pbxes.org
Password---> <pbxes-password>

Advanced Settings Section
I have not enabled "Use for telephone Numbers" because it does not work for outgoing calls.

User name --> <pbxes sip extn>



Thanks a lot to IPKall and PBXes for providing the free service.