I would do something to the effect of:
'This goes in whatever event you have once the user enters the phone number
Code:
If DCount("PhoneNum", "tblCustomers", "PhoneNum = " & Me.phoneControlName) > 0 Then
MsgBox "This phone number exists. Press OK to continue." , vbOkOnly
Exit Sub
Else
If (MsgBox "This phone number does not exist. Enter new customer?", vbYesNo) = VbYes Then
DoCmd.OpenForm "New Customer"
Else
Exit Sub
End If
End If
You should be able to follow long with what's happening there. DCount just counts up how many instances of the phone number there are.