Some things you need to know about Asterisk and DTMF.
Asterisk does have problems with DTMF - more in the 1.2 branch than the current 1.4 branch. 1.2 did not handle variable length DTMF.
No matter what you have done with your particular Asterisk setup there will be some issues that you cannot resolve!
You can however minimize what can go wrong with DTMF
In my experience there are two ways that DTMF can be somewhat reliable with Asterisk.
1. If you are using _only_ internal Analog (FXO) cards (no voip carriers or SIP gateways) than its best that you set everything (pbx and phones) to inband and set your phones to use a non compressed codec (711 ulaw)
2. if you are using other codecs (g729) and voip carriers than the best method is RFC2833. Don't mix different methods across your voip network. Your phones should use RFC2833, your gateways, and your PBX!
Most of the problems comes when people try all kinds of crazy setups.
This is not a 100% solution since DTMF problems can be caused by noisy (analog lines) and voip carriers not correctly passing your DTMF signals to upstream carriers.
You may be sending DTMF to your voip carrier using out of band DTMF and your carrier might be terminating your call via PRI. The DTMF signal will be lost!