Your mobile device does not need any internet connection or airtime before you can use the code to check your First bank account balance. Best of all, you can use this code from any mobile device that can dial a number, so this is not restricted to only smartphones.
Dial 89400# on the phone number that is linked to your First bank account and waits for the USSD code to run. If all goes well, you will receive SMS containing the balance you have on your UBA account.
N/B; make sure you use the mobile number you registered your bank account with, you can register a new number at your nearest ATM point.
Alternatively, you can use the First Monie app to check your account balance.