I have been using 2 different SIP trunks providers in Canada. (Callcentric and TieUs)
After I setup the SIP trunks, I found out one provider can go through with DTMF but another did not.
So I did a quick research on DTMF, this is what work for me.
on the peer detail of the trunk setting.
You can specified the DTMF you are going to use.

there are 3 different mode.
sip info, rfc2833 or inband.
So you can do the following
dtmfmode=inband
to specified the dtmf as inband tone

I found in most of the case, using dtmfmode=rfc2833 seems working for most provider.

Zars