Zoiper is free to use client bro.

The free version comes with ads and some other limitations, but it fulfills our requirements.
It is available on all popular platforms like: Android, Windows, iOS, MacOS, Linux.
You can check their website for more details:
Zoiper - Free VoIP SIP softphone dialer with voice, video and instant messaging :: Zoiper
Unfortunately, the newer Airtel stock ONTs are locked down to disable external softphone clients like Zoiper.
You would need to buy a third-party XPON (GPON/EPON) supported ONT and plug the Airtel patch cable directly into that ONT PON port. Then you can configure the ONT to run internet & VoIP services.
I recommend using the
TP-Link XX530v.
These days it's available on
Amazon India too, the pricing is higher there than elsewhere though!
The above mentioned TP-Link ONT's setup guide for both Airtel Fiber & Jio Fiber are available on YouTube.