Please indicate in more details what exact actions you are making with your TRM250 and what DTR signal you have in mind. I.e. TRB250 is an USB modem. Even though it is controlled over AT commands, it does not have full DB-9 connector's support.
Could you let us know all actions what are done with TRM250 device, after it is powered on? I.e. what exact AT commands you are sending to it? How call is established - from TRM side, or does TRM only answer it? Where this DTR signal is changed and how?