Let me make sure I understand your requirements.
You save the cell phone number in the database as: 12345678901.
When sending letters or emails you what the cell phone number to display as: 1 (234) 567-8907
So you are looking for a function to convert 12345678901 to 1 (234) 567-8907
You could do this using several SUBSTRING functions to pull out the various pieces of the cell phone number and add the mask fields as needed.
This could get ugly.
A better approach (IMHO) would be to store the data in the database with the mask characters.
Then when sending letters or emails you do not need to change anything.
When sending SMS you could use the REPLACE_PATTERN function to remove any non-numeric characters
REPLACE_PATTERN(BO.CellNumber, '[0-9]', '')