Sending used phone bought in India back to India from abroad
You will still have to pay customs duty. I do not think the Indian bill will be honored by customs.
Indian customs allow declaring products while leaving India
The normal practice is to declare and get a receipt from Indian customs at the time of leaving India, when somebody carries extra phone (or laptops) abroad. They can then show it to customs when they return to avoid the otherwise applicable duty on imports.
Now, in your case, you do not have that receipt from Indian customs that you brought this phone from India to US.
And second, you want to send it by post to India.
It is not possible to prove that this phone is what you brought to US and now sending back to India.
I have not heard of any case which may have skipped custom duty with same scenario.