Hello,
Zoiper Premium SIP Softphone supports some additional features that are not available in the free version. The features are Call-transfer, 3-party conference, Call recordings, ZRTP encryption, Presence and Wideband codecs (G.722, G.726, Opus, etc.). You may review and download Zoiper Premium SIP Softphone from here:
https://itunes.apple.com/us/app/zoiper-premium-sip-softphone/id787863350?mt=8