Here are the best free SIP softphone apps for Windows and where to get them.
Best Open Source SIP App: Jitsi
Jitsi
Works well on any operating system.
Time-consuming to configure properly.

Jitsi
Jitsi is a Java-built open-source instant-messaging (IM) software loaded with features.
Along with other IM features, it also allows voice and video communication through SIP.
Other interesting features include call recording, IPv6 support, encryption, and support for many protocols.

Linphone
Make Calls on Any machine: LinPhone
Linphone
Well-organized interface.
Lacks a clickable standard phone dialer.
Only supports VP8 video codec.

Ekiga
LinPhone is open-source software for Windows, macOS, and Linux, plus Android and iPhone mobile platforms.
Best Google Integration: Blink
icanblink.com
Easy Google Drive and macOS integration.
Offers a lite version for Mac users.

MicroSIP
No hotkeys or redial button.
Hold feature can be finicky.
Blink is simple and has all the features you better make voice and video calls over SIP.
It also includes a chat tool with end-to-end encryption and secure file transfers.
Available for Windows, macOS, and Linux, it’s distributed under a GPL license.
Simplest Interface: Ekiga
Ekiga
Offers multiple VoIP codecs.
Edit the source code for a personalized experience.
Fewer features than similar apps.
No version for Mac users.
Formerly known as GnomeMeeting, Ekiga is general public license software for GNOME (Linux) and Windows.
It offers all the basic features needed for good, fluid SIP communication in a clean, clear interface.
Ekiga also offersfree SIP accounts.
you’re able to use Ekiga for voice calling and video conferencing.
Best Lightweight SIP App: MicroSIP
MicroSIP
Supports dozens of languages.
Complicated set up process.
Only available for Windows.
MicroSIP is a free, open-source app that allows high-quality VoIP calls through SIP.
MicroSIP is a portable app available exclusively for Windows.