Is it possible to create firmware for button mobile phone?

Hello guys!
I’m middle fullstack web developer, but new to this RTOS and C/C++ developement.
I want to create good firmware and soft for simple button phones like this:

I looking for any info about creating firmware for button phones and people, who did it. Try to find firmware with open source code, try to find documentation, but no result. I found only Simbian OS from Nokia on github, but dont now how to compile and launch it.

Some days ago i found info about platform for creating button phones firmware: Nucleus RTOS and RTOS as a whole. Nucleus is used in most phones now.

Is it real to use FreeRTOS as base for creating firmware for button phones?
What you know and can advice me about creating firmware?
Is there starter kit or template firmware for phones?

I think FreeRTOS is used in some low cost/low power button phones, but I’m not aware of an SDK for this.

I don’t think such phones allow for adding your own applications. Their firmware is most likely a closed source package and you would have to update the entire firmware image to make any changes.
Perhaps there are exceptions but I have not aware of any.

100% their firmware id closed source.
hat’s why i’m looking for open source firmware or the basis on which I can create my own firmware.

But not any firmware needs to be done. Show “Hello world” on a screen not hard. Need support for calls and sms in base… or understand how to support it