Oncomm event is fired occasionally

Question

I'm trying to read incoming SMS

RThreshold value is set to 1

The Meassage Storage Area is "SM"

However, OnComm event is fired only when I send a command (e.g MSComm1.Output = "AT" & vcCrLf).. OnComm is fired when "OK" is returned

Why OnComm event is not fired when an incoming SMS is received?

Thank you.

Private Sub MSComm1_OnComm()
    If MSComm1.CommEvent = comEvReceive Then
        Beep
    End If
End Sub