Can not interrup the BackGround app in dialplan!

hello, all of asterisk users:
I run asterisk with tdm400P. when i call in using my hand phone, the system will play IVR and ask me to enter extension number. but i can not interrupt the background app in asterisk. it plays the IVR until finish the IVR, then i can enter extensions. if i input an extension during playing IVR, the system will forward to a wrong number or miss some numbers. for example, if i enter 890, it will give me 89 or 90, or other number. if the IVR is finished, then i enter an extension, the system run without problem. i think the TDMF tone is wrong. how can i check whether the TDMF is wrong and how ot fix it? Does anyone has such problem? please give me a hint for that.