The Win32 build of Asterisk is based on a version that ceased to be supported many years ago (maybe as many as 5). Most people will have forgotten any fixes they had to apply for that version.
The only sensible recommendation is to move to Asterisk 11 on Linux.
How are you trying to implement the conferences? If in the phone, Asterisk will not know they are conferences. If in Asterisk, for that old an Asterisk, you will need zaptel loaded, and that is only possibly on Linux.
Because it doesn’t support zaptel, that version will also have problems if the phones suppress silence. That may be what is happening in your case.