When using modems, it is often possible to setup the software and modem such that all appears to be healthy. The modem channel in ClearSCADA shows as Healthy. The modem will dial when required and establish a connection to the remote device. However, when the SCADA attempts to transmit data across the link, no data is actually sent past the modem. As far as ClearSCADA is concerned, it was successfully sent to the modem and the SCADA simply appears to receive no response from the remote device. In actual fact, the data was probably not sent by the modem at all.
Check the PSTN channel object and ensure that the Connection tab has flow control set to "Hardware RTS/CTS".
Many modems require hardware handshaking, using RTS or DTR lines connected to the modem to control the flow of information. If these lines are not used, then the modem may not work. In other cases, connection will work without any flow control and may actually not work if hardware flow control is enabled.