Have you tried Talkonaut? This is an application which is similar to Google Talk and is aimed at mobile users. As compared to other IM clients you can use all VoIP features on any of the J2ME compliant devices. You don't need to buy any extra handset for enjoying this service.

With it you will be able to make calls from your cell phone to users of Google Talk and MSN or any of the SIP phone services. You can certainly enjoy cheap VoIP calls with it. It is a very simple service with no registration required. All you need to do is download it on your mobile phone and run it. It uses the GTalk2VoIP technological service and this is what makes it a simple service for you all. Since it only supports IM therefore you would not be able to make free calls as of now. Overall it seems to be a nice service.