VB Script in ASP

[Codehammer]

Response.Write("Your Cellphone Number:<input type='textbox' onblue='validateTelephone' name='oCELLPHONE'/><br>")

I Have the Following VB Script Code:

Sub validateTelephone()
  if mid(document.frmCustomerDetails.txtTelephone.value,1,1) <>"(" then
     MsgBox "The Telephone Number must be in the Form: (000)000-0000",8,"Error"
     
        elseif mid(document.frmCustomerDetails.txtTelephone.value,9,1) <>"-" then
     MsgBox "The Telephone Number must be in the Form: (000)000-0000",8,"Error"
end if 
end sub

I Dont know where to Put This, I Tried in the Body which Gave Errors, Then I Tried in the Head in its own [script] ,[/script] and then it worked but the following error for document.frmCustomerDetails.txtTelephone.

I Know is Suppose to be request.form(oCELLPHONE) instead of document.frmCustomerDetails.txtTelephone but that doesnt Work. That is Why I Posted the Vb Code Fully.
Where is this thing Wrong? Onblur is used, then thats maybe why request is getting the error since I think request is only for Posts or Not?

Thanks