For Nigerian networks, there are some things you need to know. To make a call, send a text message, you need to always know your remaining account balance. See How to check credit or account balance on Airtel in Nigeria?:
To confirm Airtel account balance is simple, just dial *123# on your Airtel number and hold on for a text. The text messages will show you all the necessary account balance info you need to know about your airtime balance.
The service is available throughout the day and it is free of charge.