You can use fonworld from your BlackBerry. We recommend the download of one of the BlackBerry-apps you find below. Those apps offer comprehensive solutions for calling via SIP and therefore the use of fonworld.
Please note the costs for the download of the full versions charged by the respective provider.
Vippie Mobile for BlackBerry is a SIP softphone client that works on BlackBerry GSM handsets
with OS > 4.6, OS 5 and OS 6.
fg's BlackBerry SIP VoIP Client is a feature rich soft phone application for BlackBerry devices. It can register to any SIP VoIP provider or IP/SIP capable PBX using a WiFi connection in office, campus, home, hotel rooms, public hot-spots etc.