Dial digits (1 or 2) when call active/answered from custom Default Dialer in Android programmatically?

Question

Im making a Default Dialer App in Android replacing built-in phone dialer. Everything is working fine, call is attending as well as rejecting.

But when the call attends: When suppose operator asks to press "1" for this and "2" for this.

How can I send these (1 or 2) commands to operator programmatically without interrupting my custom calling screen and without going out of my custom call screen.

Things I have tried for pressing button 3:

Intent intent = Intent(Intent.ACTION_CALL, Uri.parse("tel://3"))
startActivity(intent)

But the above code prompt me a dialog "Unable to make conference call."

Thank you.