Airtel — unable to make calls on airtel international roaming

I purchased an international roaming package for inr 2499 for singapore on 12-dec-2017, valid for 30 days, unlimited incoming, 5 gb data, 100 sms/day, 500 min locals calls and calls to india. It worked till yesterday and suddenly no calls are going through. But i am continuing to receive incoming calls. I have been checking the usage through a call metering app and i haven't cross 250 min mark yet. Pls help urgently.
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