Right now there is no VoIP software per say but open source software developer PJSIP that can make VoIP a reality for Blackberry 10.
DevBlog did an interview Adam from Truphone about his app Tru App which uses PJSIP. He’s asked about the app development, coding, APIs used, and his experience working with the RIM team.
Adam does mention that Try App will be ready for Blackberry 10 when the devices launch.
Read the whole article at the DevBlog.