String strAlphabet = "АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЬЪЭЮЯабвгдеёжзийклмнопрстуфхцчшщэюяABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'-* :;)(.,!=_";
        String[] ArrayUCSCode = new String[137]{            
            "0410","0411","0412","0413","0414","0415","00A8","0416","0417",
            "0418","0419","041A","041B","041C","041D","041E","041F","0420",
            "0421","0422","0423","0424","0425","0426","0427","0428","0429",
            "042C","042A","042D","042E","042F","0430","0431","0432","0433",
            "0434","0435","00B8","0436","0437","0438","0439","043A","043B",
            "043C","043D","043E","043F","0440","0441","0442","0443","0444",
            "0445","0446","0447","0448","0449","044D","044E","044F","0041",
            "0042","0043","0044","0045","0046","0047","0048","0049","004A",
            "004B","004C","004D","004E","004F","0050","0051","0052","0053",
            "0054","0055","0056","0057","0058","0059","005A","0061","0062",
            "0063","0064","0065","0066","0067","0068","0069","006A","006B",
            "006C","006D","006E","006F","0070","0071","0072","0073","0074",
            "0075","0076","0077","0078","0079","007A","0030","0031","0032",
            "0033","0034","0035","0036","0037","0038","0039","0027","002D",
            "002A","0020","003A","003B","0029","0028","002E","002C","0021",
            "003D","005F"};
...

// формируем сообщение
StringBuilder PDUMessage = new StringBuilder();
                PDUMessage.Append("000100" + String.Format("{0:X2}", CellNumber.Length + 1) + "91");
                PDUMessage.Append(ConvertPhoneNumber(CellNumber));
                PDUMessage.Append("0008" + String.Format("{0:X2}", SMSText.Length * 2) + ConvertTextToUCS(SMSText));
// отправляем сообщение
                spGSMModem.Write("AT+CMGS=" + Convert.ToString((PDUMessage.Length / 2) - 1) + "\r");
                System.Threading.Thread.Sleep(9000);
                spGSMModem.Write(PDUMessage.ToString() + Convert.ToChar(26));
                System.Threading.Thread.Sleep(3000);

        private String ConvertTextToUCS(String InputText)
        {
            StringBuilder UCS = new StringBuilder(InputText.Length);
            Int32 intLetterIndex=0;
            for (int i = 0; i < InputText.Length; i++)
            {
                intLetterIndex = strAlphabet.IndexOf(InputText[i]);
                if (intLetterIndex != -1)
                {
                    UCS.Append(ArrayUCSCode[intLetterIndex]);
                }
                 
            }
            return UCS.ToString();
        }


        private String ConvertPhoneNumber(String PhoneNumber)
        {
            StringBuilder NewNumber = new StringBuilder(PhoneNumber.Length);
            if (PhoneNumber.Length / 2 == PhoneNumber.Length / 2.0) // число четное
            {
                for (int i = 0; i < PhoneNumber.Length / 2; i++)
                {
                    NewNumber.Append(PhoneNumber[2 * i + 1].ToString());
                    NewNumber.Append(PhoneNumber[2 * i].ToString());
                }
            }
            else // номер с нечетным кол-вом символом
            {
                for (int i = 0; i < PhoneNumber.Length / 2; i++)
                {
                    NewNumber.Append(PhoneNumber[2 * i + 1].ToString());
                    NewNumber.Append(PhoneNumber[2 * i].ToString());
                }
                NewNumber.Append("F");
                NewNumber.Append(PhoneNumber[PhoneNumber.Length - 1]);
            }
            return NewNumber.ToString();
        }