HSFModem could not build on kernel 2.6.28-11-generic

I use kernel 2.6.28-11-generic and want to install my connexant modem driver from dell. Install is success, but when build hsfconfig there is an error like below:

make[1]: Entering directory `/usr/src/linux-headers-2.6.28-11-generic'
CC [M] /usr/lib/hsfmodem/modules/mod_engine.o
In file included from /usr/lib/hsfmodem/modules/mod_engine.c:10:
/usr/lib/hsfmodem/modules/imported/include/osservices.h:356:20: error: string.h: No such file or directory
make[2]: *** [/usr/lib/hsfmodem/modules/mod_engine.o] Error 1
make[1]: *** [_module_/usr/lib/hsfmodem/modules] Error 2
make[1]: Leaving directory `/usr/src/linux-headers-2.6.28-11-generic'
make: *** [all] Error 2

After browsing find an answer:
Modify #include on modem folder xx/modules/imported/include/osservices.h in source file to #include

Modem running free in 56Kbps