I added this config in a debian 8.3 live
root@debian:~# cat /etc/usb_modeswitch.d/19d2\:1013
# Vodafone (ZTE) K3806-Z
TargetVendor= 0x19d2
TargetProduct= 0x1015
MessageContent="5553424312345678000000000000061b000000020000000000000000000000"
NeedResponse=1
NoDriverLoading=1
but after plugging in the modem it did not change mode
then I installed modemmanager and modeswitching to 19d2:1015 worked without any config changes
then I removed modemmanager and modeswitching stopped working again
It really seems that this package is required for this specific usb modem to work