Advanced Find and Replace by Column

I have attached an example spreadsheet.

Here is what I am trying to do. I have a column with 5 digit extensions and a column for 11 digit extensions. These extensions are in two different ranges. 5XXXX and 7XXXX. I need a formula that will check the values in the column and if it finds a number that begins with 5XXXX then transform it into 1312555XXXX or if it finds a 7XXXX transform it into 1312777XXXX.

I am sure this is easy to accomplish.

Thanks!