hmm....sip client from n900 works fine with SIP.
mobilevoip i believe is built for voipstunt, etc (voipbuster, etc from the same company group), and mine works with voipstunt.

Perhaps you need to try skype first. See whether it works since skype also has skype out which also have a lower rate than normal telco provider.

But before doing those above, please check the law. last I heard, some middle east country banned voip.