DTMF in 183 Session in Progress

Hi,

When somebody makes a call to HMP (linux relase ) after i send 183 Session in Progress is it possible to get DTMF  before 200 OK ? 

After i answer the call with 200 OK it works ,but i would like to be able to detect DTMF before i send 200 OK because after i answer with 200 OK billing takes place. 

I would appreciate any advice on this subject. 

Thanks  in advance