Unable to use GSM and GPRS with 3G USB Dongle on BeagleBone Black at the same time

I'm working on BeagleBone Black (Debian running on it) and I have connected a 3G USB dongle (Huawei E173u-2) with it for GPRS and sending SMS. Now I am facing a problem when I am booting my board (dongle attached). I can access GPRS service, but cannot send SMS at the same time (using AT commands from MINICOM). If GPRS service is on and I am sending AT command to 3G USB dongle, I am getting a reply Service Temporarily Unavailable. If I run command on terminal ifdown gprs(service name) ie. GPRS service is OFF, now I am able to send SMS. In SMS mode if I run command in terminal ifup gprs(service name) I am not able to get GPRS connection until I close MINICOM connection used to send AT commands to 3G USB Dongle. As I have searched about this issue, I have read a blog which says each service locks the port at which dongle is connected. Until service releases the port, other service cannot access the same port. I have tried this on Windows and found that I can access both services at the same time. What is the actual cause of this issue and how to resolve it?