voip software for sigmarion III

I have seen different posts regarding voip software for hpc 2000, but what about for windows ce 4.1. I tried the hacked skype version but it fills completely the available ram on my device. I would like to know if other sofware is available which uses less resources from the system. Maybe voipster .... but where can I download the right files for sigmarion III?
Which are your options?

how does skype fill the ram?

I mean when installing skype it must be done in the sig (main unit) and not in the sd card. So the memory of my sig is almost full and I do not want this, is there other voip program smaller or that can be installed in the sd card?