Airtel for a while have had some plans which gives double of what you subscribe for called 1+1, but the
sad thing was not all SIM are eligible to subscribe to
this plan. If you are eligible you get 4GB data for 1500 naira if not eligble you get just 2Gb for same price, so getting eligible was something everybody wanted.

How to get your Airtel SIM to be Eligible

Step 1.
Dial *440*440# and if it works for you then you will get a message that: “You are ELIGIBLE!Dial *440*161# for the Unlimited 1+1 offer at N1,500 to get 4GB valid for 60days or dial *141*1# for other BB1 + 1 offers“

Step 2.
If the first method doesnt work for you then the last
resort which will always work is, to call customercare and tell them you want to enjoy the 1+1 plan and surely they will make your SIM eligible.