Why can’t i make calls or check my account balance for the past 3 days now

I can browse check balance and send messages but can't make calls

I can't make call with my MTN line even when I want to check my airtime balance it can't even show me

I subscribed for 200gb plan at 37,500 on my router on 16th july when I had a balance of 50gb. I checked my balance after and the 200gb reflected with additional 10gb...

I have momo recharge bonus but I can't use it. I am told that my account balance is not sufficient to make calls