I tested my fax extension with some fax machines and they worked. I
then pointed a client at it and the first two fax machines they tried to send to failed with the following error.

WARNING[-1][C-00022893] res_rtp_asterisk.c: RTP Read too short

I am wondering if there might be some incompatibility with some fax machines and not others. Or, is there a setting that I am missing.

Note that even though it is a warning

Is there some way to increase the verbosity of sendfax? Is there some other place I should be examining?

I am using Asterisk 11.23 on a NetBSD system. My extension looks something like this:

exten => sendfax,1,Verbose(0,FAX ${faxfile} to ${destination})
same => n,Set(FAXOPT(headerinfo)