Workbooks.Open method failing to run

I'm trying to change a subroutine that I've been using for a while into a function, so that it can be called directly rather than from a separate subroutine. Here's what I have at the moment:

Function FindCode(strPhoneNum As String) As String
    'Sorts through the data in the Master Inventory that may contain a phone number; when it finds the
    'number it's looking for, it returns the first 7 digits of the "account name" as the coding string,
    'and exits the function.  If it doesn't find anything, it requests user input from another source.

    'Set Object References
    If IsRepOpen("CL Master Long Distance Inventory 2012.xls") = False Then
        Workbooks.Open Filename:="\\files1\TelcomPublicShare\CL Master Long Distance Inventory 2012.xls", ReadOnly:=True
    End If
    
    Set wbTelcom = Workbooks("\\files1\TelcomPublicShare\CL Master Long Distance Inventory 2012.xls")
    Set wsList = wbTelcom.Worksheets("Outbound")            'Identify tab with number inventory.
    
    'Validate Phone Number
    Do While strPhoneNum Like "*[A-Z]*" Or Len(strPhoneNum) <> 10
        strPhoneNum = InputBox(prompt:="Phone number should only be 10 digits long and" & vbCr & _
            "include no alpha characters or punctuation.  Please input valid number:" & vbCr & _
            "Example: 1234567890", title:="Invalid Phone Number", Default:="XXXXXXXXXX")
    Loop
    
    wsList.Activate
    
    'Find the phone number in the Master Inventory; request valid billing code if number is not in inventory.
    On Error GoTo ErrorHandler
        FindCode = Left(Cells(2, Cells.Find(strPhoneNum).Column), 7)
        Application.ScreenUpdating = True
        ThisWorkbook.Activate
    Exit Function

ErrorHandler:
    Application.ScreenUpdating = True
    FindCode = InputBox(strPhoneNum & " is not in the" & vbCr _
        & "Master Inventory.  Please input" & vbCr _
        & "the seven-digit billing code.", "No Valid Code", "XXXXXXX")
End Function

The problem is that when I run the Workbooks.open method from the function, it doesn't do anything. It loads all the property data for the file into wbTelcom, but the file doesn't open up, and the object seems to refer back to ThisWorkbook instead.

This only seems to be a problem if I enter macro processing through the function; if I use a command button that calls a standard subroutine with the method called there, it works fine. Is it some sort of problem with trying to call the method from a function? Or did I screw up the syntax somewhere?

As a user-defined function from Excel. A simple '=FindCode(B2)'

Picking up on the lead you were putting there, I tried running it from the Immediate window in the editor and that did work. Still no luck with the formula being run from the worksheet though. Even if I have the workbook open from the start, it doesn't set the wbTelcom object to the appropriate reference for some reason.

Ah, didn't know that.

So, that explains why it won't open the workbook properly when I have it closed. Does the same apply to activate worksheets in other workbooks? If it does, that would explain both of my problems.

The same reason that I needed to open the workbook; my Master Inventory file is in another workbook from the worksheet where I've got the data that I'm searching for, so I need to open it and switch over in order to find the billing code used for that number.

I'll probably just go back to the original method I was using and not worry about creating a function, at least for now.