@Ameen In starting I was thinking that for using landline we have to connect it to only bsnl's
ONT modem (ont's landline port). But then I realised that most of the personal
routers also have
adsl's Rj45 port (landline port).
So we can use the landline through router's landline port (Rj45 port) along with the internet by LAN by using bridge mode technique and also need to properly configured the VoIP and internet WAN settings in router.