There are lot many VoIP apps are available in market and all have their own different feature. Below are some VoIP apps for Android mobile.
7. Groove IP Lite
10. TiKL Touch Talk Walkie Talkie
11. Google Voice
Windows Mobile VoIP Apps, Mobile VoIP Apps for the Blackberry, Android-only Mobile VoIP Apps, iOS Mobile VoIP Apps are also available online, just Google it and you will find a bunch.