Android - how to merge calls (GSM/SIM) to conference (InCallService)

I'm trying to merge two GSM calls into conference programmatically.

In Call class is method conference() but I'm not sure how to correctly work with it. I tried several ways but nothing work successfully.

I have class CallService (implements InCallService)

And in method override fun onCallAdded(call: Call)

I tried something like this:

    activeCall.hold()

    secondCall.answer(VideoProfile.STATE_AUDIO_ONLY)

    secondCall.conference(activeCall) 
    secondCall.mergeConference()

But it will not create a conference call (the first call is set as busy). I tried several changes in that code but nothing work.

I also find this how to merge call programmatically in android- conference-call and this How to merge Call programmatically while other call is running (Conference call) but it's not helped (and it's a little old)

Thanks for help