Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Textnow.com - It lets you text your friends, aka my only source of happiness
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Talk360 - Talk360 is a best-in-class communication utility that offers a convenient way to connect with loved ones globally.
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
Molly (Signal fork) - Molly is an independent Signal fork for Android