When you type a seven-or-ten-digit phone number into an Excel cell that uses the phone number format, Excel will automatically change the way the number looks so that it appears as a phone number ...

Combine an Excel custom format with data validation and remove the pressure for input perfection from personnel by letting Excel do the work.

If you Can't type numbers or formulas or enter data into Excel cells then it might be a problematic Add-in causing the problem. Here are some working fixes that may help.