|The following step-by-step instructions worked for me. If they don't work for you, you'll have to tell me exactly where things don't work as I described.|
Since I can't see what you are doing from where I'm sitting, nor do I know what your level of knowledge is, I'm going to be very specific. Don't take anything I say as an insult, I'm only trying to help.
The method I am suggesting uses what is known as a "BeforePrint Macro". The BeforePrint macro is a feature available within Excel VBA that allows you to automatically run a VBA macro immediately after Print or Print Preview is chosen.
These step-by-step instructions should explain how to place the BeforePrint macro into your workbook.
1 - Highlight the code at the bottom of this response. Either right-click and choose Copy or hit Ctrl-C. This will place a copy of the code on the Windows Clipboard.
2 - Open the workbook with the Summary and Telephone sheets.
3 - Hold down the Alt key and press F11. This should open the VBA editor. The title of this window should end in [Sheet1 (Code)]
4 - On the left hand side of the window you should see a pane entitled "Project - VBAProject". Assuming that the workbook where you want the custom header to be placed is the only one open, you should see a list of the sheets in the workbook. The workbook that I tested this in causes the list to look something like this:
If you have more sheets with different names, you should them all, with ThisWorkbook at the bottom of the list.
5 - Double-click the word ThisWorkbook. The title bar at the top of the window should now end with the string:
6 - Right-click in the big empty pane on the right hand side of the window and choose Paste. This should place the Code you copied into that pane.
The pasted text should contain some blueish text and some black text, but no red text. If it contains any red text something is wrong. Other than the color differences, the code should look exactly as it does below.
7 - Click the Red X in the upper right corner of the VBA Editor window to close it.
8 - Back in the Telephone sheet, either Print it or use Print Preview. Whatever value is in Summary!A1 should be placed into Center Header.
If you change the value in Summary!A1 and choose Print or Print Preview, the CenterHeader will change to that new value.
If you are unable to see the results I described in any of these steps you'll have to tell me exactly what you see (or don't see). Simply stating that it's "not working" will not allow me to help you since I can't see what you are doing.
Private Sub Workbook_BeforePrint(Cancel As Boolean)
Dim rFormulaCell As Range
Set rFormulaCell = Sheets("Summary").[A1]
ActiveSheet.PageSetup.CenterHeader = rFormulaCell