Example: If you don't have enough funds in your account when your next DID invoice is due to be charged, your account balance will go into a negative balance. If you have not recharged your account within 14 days of your account balance going in to debit, your account will be terminated.
Crazytel bills in real-time, however, it is not uncommon for your account to go into a negative balance by a few cents if you were on a call, while you run out of funds. Usually, it is only a tiny amount. If you are on a call at the same time your balance is exhausted, your call will be terminated.
While your account is in a negative state, you can not make or receive calls.