It is not something we can fix from FOP2... as FOP2 controls the PBX and not the phone. There is an AMI command sendDTMF that we use.. now, if the dtmf is really being sent or not, or recognized, is something beyond FOP2. As the sendDTMF is an 'external' command, it might fail, as it is not directly in the media/flow between the two endpoints. Not sure in your diagram if the call is actually bridged with "something" when you enter the pin code, the sendDTMF requires the call to be bridged so it knows the destination channel to send the dtmf to... if there is no destination/bridged channel, then it will do nothing.
You can start FOP2 in debug mode and look for the DTMF commands being sent, if they have a proper destination channel, etc..
http://support.asternic.biz/?/selfhelp/view-article/how-to-debug-fop2/10