ATS11 command not recgonized

Hi,

I have recently purchased a Dynex USB modem and using it on
Ubuntu Gutsy with 2.6.22-14 kernel. The following is the output after
hsfconfig

Config for modem unit 0: /dev/ttySHSF0
       Device instance: 0-USB-0572:1301
       HW profile name: hsfcadmus2
       Registration ID: A42A-AD9D-9B03
       License key    : FREE
       License status : FREE (max 14.4kbps data only)
       Current region : USA (T.35 code: 00B5)

The /dev/modem alias (symlink) points to ttySHSF0

I have no problem dialing with the problem nor using commands like
ATS7. However, command ATS11 doesn't work properly. The console responds
with OK but when I listen to the dial tone, the DTMF tone duration remains the
same.

Has anyone else experienced with a similar problem and have a solution for this?

Thanks for your help,
Gary