Xiaomi Mi4i is mid range phone launched in 2014 and was sold till last year in India. Mi4i was launched to boost the slow sales from Non-LTE version of Mi4 model launched in India.
This phone doesn’t has VoLTE phone till now even while the lower priced redmi 2 prime got volte push. Mi4i is having snapdragon 615 processor which does supports VoLTE however they are not giving the VoLTE update.
Because this phone was launched only and only in India market officially. Not launched officially even in home market of china.
So that means no driving force for Xiaomi to push it as only one market would be affected at the most. India is also not that serious market for Xiaomi in comparison to its core China market. Another reason maybe that the no of handset sold are also too low.
Many user are expecting VoLTE update in MIUI 8 update however some of beta testers have confirmed it as very remote possibility.
So guys either use Jio4gvoice app
or use idea,airtel/vodafone
or better exchange your phone at Flipkart during offer period instead of the endlessly wait for VoLTE update which is not gonna happen in my opinion.