Q.  How I can Add Prefix Number +91 before mobile contacts in MS Excel?

Ans: If You want to add a prefix number before your mobile contact please follow these steps-
Step-1: Select The cells/columns/rows 
Step-2: Right Click
Step-3: Select option Format Cells
Step-4: Choose option custom
Step-5: Select general
Step-6: Add a value +910000000000
Step-7: click OK

Now your problem have solved: for more any query you can call me +91-9953216127, +91-7053074864