Well,Folks I haven't left BSNL fully for now,even after switching over to ALLIANCE BROADBAND (Cable Broadband,Kolkata)network service.
I just went to the local BSNL exchange office and 
surrendered the Broadband connection only,but keeping back the voice/telephone landline.
It is under 
General India Plan.
I have the
 option to revert back to BSNL if anything FEASIBLE occurs relating to BROADBAND bandwidth + cost,and surrendering the cable broadband.
For this decision I am really Thankful to 
@topgear & 
@whitestar_999 ,for their valuable advice ,whenever I intended to switch from BSNL to Cable Broadband services,a year or two ago,as discussing in this Forum.