I'm trying to reliably detect whether there is a fax machine on an incoming channel. Right now, it works nicely with the following dial plan: answer spandsp_start_fax_detect sleep 5000 spandsp_stop_fax_detect [... no fax there, so now do stuff that has nothing to do with fax ...] If the remote end is a PSTN fax, spandsp will fire an event upon detecting the CNG tone, and I will react accordingly. However, if the remove end is a T.38 fax, which will immediately re-invite after answer, then the following problem occurs: FreeSWITCH does not answer the re-invite (just says 100 Trying and nothing follows). Even though the remove end fax transmits the CNG tone afterwards, it is not detected by spandsp. I have recorded the call to make sure the CGN tone is there, and it is there. I have even played this tone into another phone call, where it is successfully detected by spandsp. All I can guess is that the non-answered re-invite might block spandsp from detecting the tone? Greetings, Roland