Blink

Blink is the best real-time communications client using the SIP protocol.
You can use it with many SIP providers, on the LAN using Bonjour and with SIP2SIP, a free service.

OSX Integration (iCloud, iTunes, Address Book, Keychain, Voice Over)
Clear audio using Opus codec optimised for full spectrum audio
Telephony compatible codecs: AMR, G.711, G.722
Video calls using H.264, VP8 and VP9 codecs
Acoustic echo cancelation
Presence (SIP SIMPLE RLS and XCAP protocols)
Chat and File Transfers sessions (MSRP protocol)
Instant Messaging (SIP Message)
Screen Sharing (VNC protocol)
Multiparty conferencing (audio, chat and file transfers)
End-to-end chat encryption (OTR protocol)
End-to-end audio encryption (ZRTP protocol)
Corporate directory services (LDAP protocol)
OpenPGP support for SIP messaging
Compatible with telephony applications (dialpad, call hold, call transfer)
Fine-grained settings for SIP experts

Blink does not collect any data. Contacts accessed by Blink from system address-book are not uploaded to any servers.