yes, no doubt they are very harmful and should be banned, at most 1 free hour and that too before 10 pm will be nice, or 1-2 hours only on weekends, because by this there will not be excess of anything, and secondly many students living in hostels have to call their parents at home. and same way some one who is doing job in some distant city also needs to talk to his family. but again not more than 1 hours daily, or 2-3 hours per week be allowed.

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"The only necessary thing for the triumph of evil is for good men to do nothing."
--Edmund Burke (1729-1797)