[Sim800]download file by FTP

Hi,
I am using AT command from FTP part of SIMCOM Sim800 to download a file.
These are the log I got from terminal:

send: AT+SAPBR=3,1,"Contype","GPRS"

OK
send: AT+SAPBR=3,1,"APN","m-wap"

OK
send: AT+SAPBR=3,1,"USER","***"

OK
send: AT+SAPBR=3,1,"PWD","***"

OK
send: AT+SAPBR=1,1

OK
send: AT+FTPCID=1

OK

send: AT+FTPSERV="***"

OK
send: AT+FTPPORT="21"

OK
send: AT+FTPUN="***"

OK
send: AT+FTPPW="***"

OK
send: AT+FTPGETNAME="test01.txt"

OK
send: AT+FTPGETPATH="/"

OK
send: AT+FTPGETTOFS=0,"test01.txt"

OK
send: AT+FTPGETTOFS?

+FTPGETTOFS: 1,0,0

OK

+FTPGETTOFS: 0,0

----------------------------------------------------------------------
As the last respond, it said "During the process" (1,0,0) then after few seconds it is "Not in the process" (0,0).
The file size is 15bytes.
Could someone please help me to solve this.
Thank you all!

On the other hand, when I replaced the command AT+FTPGETTOFS by AT+FTPGET, it worked well:
These are the commands I tried:

send: AT+FTPGET=1

+FTPGET: 1,1

OK
send: AT+FTPGET=2,15 //download 15bytes per package

+FTPGET: 2

....

Please note that:

FTPGETTOFS: download and store file in File system of Sim800
FTPGET: get and print out each package.

Thank you!