"Level 2 halted" error message using ldattach with mux type GSM0710

I have a 3G modem on my BeagleBone Linux system that supports GSM0710 multiplexing (i.e. can create >= 2 virtual serial ports that connect to the physical modem port, which can then be used to run ppp on one, and AT commands the other). I'm trying to use the ldattach command to get this to work, as described in https://www.kernel.org/doc/Documentation/serial/n_gsm.txt . So I run these commands as root:

ldattach -d GSM0710 /dev/ttyO2             
ldattach: opened /dev/ttyO2                                                     
ldattach: set to raw 0 ---: cflag=0x1cb2                                        
ldattach: line discipline set to 21

Then create some virtual serial devices with:

MAJOR=cat /proc/devices |grep gsmtty | awk '{print $1}
for i in seq 1 4; do
	mknod /dev/ttygsm$i c $MAJOR $i
done

However, when I try to connect to one of the /dev/ttygsm ports, I always get this error "can't open /dev/ttygsm1: Level 2 halted". Does anyone know what this error means, and how to get the above muxing scheme to work? Thanks, Fred