Similar to Gizmo5

Inter-Asterisk eXchange

Jami (software)

Jitsi

H.248

Messenger (Meta)

Media Gateway Control Protocol

Mumble (software)

OSIP

Skype

Tango (software)

TeamSpeak

Telecoms & Internet converged Services & Protocols for Advanced Networks

Tox (protocol)

Wide-coverage Internet Repeater Enhancement System

Asterisk (PBX)

Ekiga

Empathy (software)

KakaoTalk

Google Hangouts

Softphone

Discord

Viber

Line (software)