Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Topia - Create amazing wallpapers for your iPhone
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Reminder - A better way to keep track of your reminders 📝
Sipdroid - Free and open-source SIP/VoIP Client
Geneva - An all-in-one communication app for groups & communities