[Freeswitch-users] fax tone detection failed

[Steve Underwood]

On 04/15/2011 01:32 AM, Christopher Rienzo wrote:
> 2100 Hz is the ANS tone from the receiving side of the fax call.  
> That's what I detect on my outbound calls.
An answering machine sends 2100Hz, but its very unreliable to rely on 
hearing it. In spandsp there is a module which does a much more thorough 
job of detecting a FAX machine. It will accept the V.21 preamble, which 
normally follows the 2100Hz tone, as a substitute for the 2100Hz itself. 
That way a call which starts passing audio a bit late (which they often 
do) still results in proper FAX detection. We need to expose that 
functionality in FreeSwitch, but have not done so yet.

Steve