Rather than Cusstom Format, format the cell as tet and enter the - manually:

1-2-3-4-5-6-7-8-9-0-1-2-3-4-5-6

Cheryl,

Excel will truncate(?) numbers longer than 15 digits, as you've seen.
Best to format as text. I'm sure someone will come up with a VBA
solution for you, but it's still text. Access provides a "mask" that
would put in the dashes automatically, but Excel won't.

Beege

Excel's specifications are such that you cannot enter a 16 digit *number*.  You
can only enter the value as text.

Since it is a pain to enter the text with the hyphens, I would suggest

1.  Preformat the data entry fields as TEXT.
2.  Enter your 16 digit number.
3.  Run this UDF
    =InsHyphens(cell_ref)

To enter the UDF, <alt-F11> opens the VBEditor.  Ensure your project is
highlighted in the Project Explorer window, then Insert/Module and paste the
code below into the window that opens:

==========================================
Option Explicit
Function InsHyphens(str As String)
Dim i As Long
Dim Temp(15)
Dim s As String

For i = 0 To 15
   s = Mid(str, i + 1, 1)
   If IsNumeric(s) Then
       Temp(i) = CLng(s)
   Else
       InsHyphens = CVErr(xlErrValue)
       Exit Function
   End If
Next i

InsHyphens = Join(Temp, "-")
End Function
========================================

As written, the code will give valid output if your string contains 16 or more
digits.  It will give a #VALUE! error if you have fewer than 16 digits, or if
any character of the first 16 is not a digit.

You don't have to use a UDF, you could write a formula of the type:

=LEFT(A1,1)&"-"&
MID(A1,2,1)&"-" &
MID(A1,3,1) & "-" &
...

and so forth until you have handled all 16 digits.

It was easier for me to write the UDF.

--ron

If you're trying to eliminate the need to key in all those dashes, you can
try a text formula in an adjoining column that will display the data as you
wish.

It's a *long and cumbersome* formula, but doing it once and copying the
formula containing column for future use may be a viable option for you, as
opposed to having to enter all those dashes.

Enter the original values in Column A, *preceded* with an apostrophe.
This makes the entry text, and allows XL to *accurately* display the 16th
digit.

Then in B1 enter this formula:

=LEFT(A1)&"-"&MID(A1,2,1)&"-"&MID(A1,3,1)&"-"&MID(A1,4,1)&"-"&MID(A1,5,1)&"-"&MID(A1,6,1)
&"-"&MID(A1,7,1)&"-"&MID(A1,8,1)&"-"&MID(A1,9,1)&"-"&MID(A1,10,1)&"-"&MID(A1,11,1)
&"-"&MID(A1,12,1)&"-"&MID(A1,13,1)&"-"&MID(A1,14,1)&"-"&MID(A1,15,1)&"-"&MID(A1,16,1)

Copy down as needed.

I am assuming you want the 16-digit "number" you enter to be changed within
the cell it was entered in. There are two possibilities to your request;
both assume the cells are formatted as Text (which, as has been pointed out,
is required so as not to lose the 16th digit)...

1) You already entered the numbers and want to change them. If this is the
case, you can select those already filled in cells and run this macro...

Public Sub InsertDashes()
 Dim C As Range
 For Each C In Selection
   If C.Value Like "################" Then
     C.Value = Format(C.Value, "@-@-@-@-@-@-@-@-@-@-@-@-@-@-@-@")
   End If
 Next
End Sub

2) You want to type in the 16-digit number and have it change to the format
you want when you enter it. Add this Worksheet Change event to the code
window for the sheet where you will be entering your "numbers" (right-click
on that sheet's tab and select View Code from the popup menu)...

Private Sub Worksheet_Change(ByVal Target As Range)
 Const ColumnToChange As String = "A"
 If Target.Column = Asc(ColumnToChange) - 64 Then
   If Target.Value Like "################" Then
     Application.EnableEvents = False
     Target.Value = Format(Target.Value, "@-@-@-@-@-@-@-@-@-@-@-@-@-@-@-@")
     Application.EnableEvents = True
   End If
 End If
End Sub

Rick