I’m not sure why are you using iaxmodem… I use it a few years ago with Asterisk 1.4
In Asterisk v11 fax is managed using res_fax. Please see https://wiki.asterisk.org/wiki/display/AST/Asterisk+11+Application_ReceiveFAX_res_fax
You only need download, compile and install the spandsp lib for your distro
(fax depend on it) and then recompile Asterisk (if you don’t have this resource module already).
We currently receive fax via g.711 ir SIP (via T.38), convert it into PDF
and send via email. All with Asterisk v11 and OpenSource software. I can send you our scripts if you want.