Gsmcomm can open port but return no phone connected

Question

I use GSM modem seimen to send SMS. I use library Gsmcomm to connect to modem Modem plug into PC by USB Serial Port

My problem is i can open port with no error:

GsmCommMain comm = new GsmCommMain("COM20", 19200, 300); 
comm.Open();

but after that i call send message it though error "no phone connected"

pdu = new SmsSubmitPdu("test", "0965432341", "");
comm.SendMessage(pdu);

I was checked in computer device management,it exits COM20 I download this source and running too but with no luck https://www.codeproject.com/Articles/20420/How-To-Send-and-Receive-SMS-using-GSM-Modem-2

Anybody who faced this problem, help me fix this.

thanks