I am getting a strange error when I try to build my solution. The error occurs when I am calling the oGetHeaderValue function and passing the parameters.
Dim oGetHeaderValue As New clsGetHeaderValue
Dim returnString As String
returnString = oGetHeaderValue(strInvoiceNumber, strOrderNumber)
You're calling your instance
oGetHeaderValue as if it's a method. It looks like you probably meant to call a function on it instead but missed out that bit.
So maybe your code should be:
Dim returnString As String = oGetHeaderValue.YourMethod(strInvoiceNumber, strOrderNumber)
YourMethod is whatever method you wanted to call.
And just to clarify after reading your question again,
oGetHeaderValue is not a function, it's an instance of a class that might contain functions and subs etc.