entering telephone numbers

If the country code is the same for all, then

="01"&Substitute(A1,Char(32),"") or ="01"&Substitute(A1," ","") copied down... where 01 is the country code A1 contains first entry.

If the country codes are not the same we will need to know how you would determine what country code applies.

To copy down, enter the formula in first cell, then click, hold and drag down the little black square at bottom right corner of cell....