DTMF with MISDN

I’m having some problems getting outbound DTMF tones working properly using Asterisk + mISDN + an OpenVOX card.

Basically calls outbound into automated systems won’t register key presses properly.
I phoned my mobile phone from our phone system, and when I held a key, I get only about a 1/3 of a second ‘blip’ on the line. I would guess this is why the receiver cannot register the keypress properly?

What I’m confused about is which settings I need to alter to fix this problem. In /etc/asterisk/misdn.conf, I have the following setting but I can’t find any documentation as to what it does!

; ; Either if we should produce DTMF Tones ourselves ; senddtmf=yes

If it’s set to no, will something else produce them? The network? Some other part of Asterisk?

Where else do I need to look? (other DTMF settings seem to be specific to Zaptel or SIP which I don’t think we’re using)

Thanks!

Versions:
[ul]Asterisk: 1.6.0.6
mISDN: 1.1.9.1
Card: OpenVox B800P
Kernel: Linux 2.6.28.8
[/ul]