asterisk users - Dec 2005 - Zultys ZIP2 + asterisk + DTMF on other end? (i.e. ivrs, autoattendants, etc)

Hey all, I have a bunch of Zultys ZIP2 phones, and they work fine with
asterisk except on one point, when I make an outgoing call via PSTN/Zap, the
call connects & all is fine, but if I try to enter in any DTMF tones to
navigate a menu at the receiving end, the tones are never recognized by the
receiving attendant, any ideas on what needs to be done for the system to
send the tones after the call is established? this is only happening on the
ZIP2 phones, I have several others that work normally.

Thanks in advance