Idle means not in use. (not days but minutes!)
Actually above ad of Airtel is more or less fake!
Tower contacts mobile only when there is incoming phone or incoming SMS or internet activity. Otherwise tower does not have to contact mobile anyway.
It is actually reverse, mobile is supposed to remain in touch with nearby tower. Which mobile does anyway even if its idle. Just like PING in internet world. Mobile always keeps PINGing nearby tower - "Hey I am here, let me know if there is a call or SMS!". This PINGs are sent frequently
So the above ad is nothing but marketing gimmick.