Так что, в мануале напутали (SIM300_ATC_V2.00.pdf, стр.7-8)?
Цитата
Software flow control sends different characters to stop (XOFF, decimal 19) and resume (XON, decimal 17) data flow. It is quite useful in some applications that only use three wires on the serial interface. The default flow control approach of SIM300 is hardware flow control (RTS/CTS flow control), to enable software flow control in the DTE interface and within GSM engine, type the following AT Command: AT+IFC=1, 1
This setting is stored volatile, for use after restart, AT+IFC=1, 1 should be stored to the user profile with AT&W.
По умолчанию контроль аппаратный - AT+IFC=2,2.