Whenever you enter anything into Excel the format of the cells is set to "General". This means that numbers will be aligned to the right and text will be aligned to the left.
As part of that formatting, Excel will delete any 0's at the start of a number – since numbers do not start with a 0.
When entering telephone numbers this can be a problem, as many phone numbers begin with a 0!
However, there is a quick and easy way to combat this!
If it is for a range of cells, the quickest method is to select the cells first i.e. a Column.
Press CTRL+1 on the keyboard to bring up the Format Cells dialogue box.
From the Number tab select Text
Click OK or hit Enter on the keyboard.
When you type a number starting with a 0 the data will be entered correctly.