Это сработало для меня, используя GsmComm:
public string SendUssdRequest(string request)
{
string data = TextDataConverter.StringTo7Bit(request);
var asPDUencoded = Calc.IntToHex(TextDataConverter.SeptetsToOctetsInt(data));
try
{
IProtocol protocol = _comm.GetProtocol();
string gottenString = protocol.ExecAndReceiveMultiple("AT+CUSD=1," + asPDUencoded + ",15");
var re = new Regex("\".*?\"");
int i = 0;
if (!re.IsMatch(gottenString))
{
do
{
protocol.Receive(out gottenString);
++i;
} while (!(i >= 5
|| re.IsMatch(gottenString)
|| gottenString.Contains("\r\nOK")
|| gottenString.Contains("\r\nERROR")
|| gottenString.Contains("\r\nDONE"))); //additional tests "just in case"
}
string m = re.Match(gottenString).Value.Trim('"');
return PduParts.Decode7BitText(Calc.HexToInt(m));
}
catch { }
finally
{
_comm.ReleaseProtocol();
}
return "";
}