hi sufi,

is your usb modem installed properly?
i suppouse that it is found in 'lsusb'.
ssh to your rasberry.
ls /dev/ttyUSB*
is there anything (like ttyUSB0 or ttyUSB1 or so)?
if it is not,
did you modprobe it?
for your dongle command should be
modprobe usbserial vendor=0x0af0 product=0x5000
if you have not installed modprobe, just 'apt-get install' it.
if it is all ok and ttyUSB is located in /dev,
is dongle properly set in
nano /etc/asterisk/dongle.conf
near the bottom of dongle.conf find paragraph [dongle0]
uncomment and set path for audio and data (audio=/dev/ttyUSB0 say..).
if you are not sure for ttyUSB device, leave audio and data commented and uncomment and enter
imei and/or imsi numbers.
at least imei you will find printed somewhere on your dongle.
after restart all, log in to asterisk cli
asterisk -rvvvv
from cli type
dongle show devices
your dongle should be listed.
if it is not, check again all settings.
maybe your dongle is locked... google 'unlock 3g dongle'.
if dongle is listed, 'exit' from cli and check settings in raspbx gui.