Sale has ended but you can still get this app.
Download Calls+ Now
Calls+

Official Description

Audio call, video call and message.
UDP, TCP and TLS transports.
G711, G722, G729, GSM, iLBC, SPEEX, OPUS codecs.
Call transfer, recording, sending of DTMF, echo cancellation.
Intergration with phone contacts.
Supports calling over 3G and WIFI.
Supports Push Notification, run app in background.
Supports multi SIP accounts.