Or, alternatively, if brevity is an issue, you could copy/paste this function to a module in Northwind:
Function removeit(thestuff As String) As String
'*******************************************
'Name: removeit (Function)
'Purpose: Remove all non-numeric characters from a string
'*******************************************
Dim strhold As String, intLen As Integer, n As Integer
strHold = RTrim(thestuff)
intLen = Len(strHold)
strHold = ""
For n = 1 To intLen
strHold = strHold & IIf(IsNumeric(Mid(thestuff, n, 1)), Mid(thestuff, n, 1), ""

Next n
removeit = strHold
End Function
…and test it using this query to view the reformatted phone numbers in (table) Customers:
SELECT CompanyName, Phone, removeit([phone]) AS widget
FROM Customers;