FreeSWITCH is fairly old (2012-05-30), though I haven't seen many
fax-related changes lately.
I'd love to try git HEAD but it's fairly hard given tat new Debian packages
complately change filesystem structure.
The Cisco uses User-Agent Cisco-SIPGateway/IOS-12.x, no idea what model it
is.

Hope there is any way to debug this without upgrading to git HEAD.