MobileVoip is a free iPhone calling app which lets you make (free) national or international voip calls directly from your iPhone. You can save on your Mobile Phone costs by simply installing the MobileVoip app. Select any contact from your contact list which is accessible in the app and start your call using MobileVoip anywhere, anytime.
MobileVoip operates over multiple internet connection options (3G, WiFi, GPRS, Edge, UMTS) directly from one integrated contact list. Install MobileVoip now and start making free 3g calls and WiFi calls today; no need for expensive calling plans anymore!
Lots of VoIP providers are supported. Make sure you have an account already, before using the MobileVOIP app.
More info at Developer's page (below)
See the Screenshots below