Do I need to set hardware flow control when using a GSM Modem?

This depends on the DCE (Data Carrier Equipment), the device to which you are connected. If this device requires hardware flow control, you’ll need it too. In most cases, hardware flow control is required; that’s why it is enabled by default.

NOTE: when using a Windows Telephony device, hardware flow control settings are inherited from the device settings and do not need to be set.