To remove the dashes from phone numbers (e.g. change 206-456-789 to 206456789):
=SUBSTITUTE(A1,"-","")
To remove dashes and add digits to the beginning of phone numbers (e.g. change 206-456-789 to 1206456789): NOTE: This should be pretty useful in other situations too and should handle small additions better than creating a new cell/column with what you need to add and then Concatenating everything together.
=1&SUBSTITUTE(A1,"-","")Reference