Can anyone enlighten BSNL 108 BASE PLAN Kolkata circle benefits and charges

I have a bsnl number which had 101 balance in feb 2020 and plan validity till feb end or march 1st week and yesterday i checked it was showing same balance with 108 base plan activated and validity till 1 may 2020. i never use that sim then how come plan validity date is getting extended ?

what call or data benefits I am getting under 108 base plan ?

PS i dont remember if i recharged that using amaz offers in feb or march.