[Freeswitch-users] request SDP in 200 OK

Hi list, I want to receive faxes with T.38 and spandsp (app spandsp). The INVITES for the calls come with or without SDP-headers. If the SDP-header is in the INVITE to freeswitch, Freeswitch can receive the fax without problem. But if FS gets an INVITE without SDP it can not receive the call and ends with error code 'MANDATORY_IE_MISSING', because FS doesn't send an SDP-offer in the 200 OK message. How can I achieve this? Is it late-negotiation? Benjamin