Please help me configuring Action VOIP on Nokia N900.

Here is the configuration mentioned on their website

Software configuration

General
SIP port : 5060
Registrar : sip.actionvoip.com
Proxy server : sip.actionvoip.com
Outbound proxy server : leave empty
Account name : your actionvoip username
Password : your actionvoip password
Display name/number : your actionvoip username or voipnumber
Stunserver (option) : stun.actionvoip.com


Codecs
G.711 (64 kbps)
G.726 (32 kbps)
G.729 (8 kbps)
G.723 (5.3 & 6.3 kbps)
GSMFR (13.2 kbps)

If you have audio problems:
Use a STUN server (e.g. stun.actionvoip.com) with port 3478 (if supported by your device)
Use the G.711 codec

P.S. There other similar voip services like Just VOIP, Free Call