Its called Borrow Me Credit.
GLO allows you to borrow up to the highest amount you've recharged. You need to have been a loyal and consistent subscriber on Glo for at least 4 months before you can borrow airtime from GLO.
To borrow airtime from GLO, follow the steps below:
Step 1: Create a 4 digits pin. On your GLO sim, dial *321# then create a 4 digit pin of your choice e.g 4535
Step 2: Borrow the airtime. Dial *321*pin*amount you want to borrow#. E.g
The amount you'll receive is less 10% service charge. That is, if you request for N200 credit, your phone will be credited with N180 while N20 goes to Glo as their service charge.