Hello, I'm a voip specialist with 10years of experience working on the level2/NOC department of a US voip company; I can install, troubleshoot and customize many sip devices including softphones, ATAs, Ip-phones and PBX systems like Asterisk/FPBX. I read your project description, and I'm wondering if you're using a regular fax machine attached to an ATA, or if you have a dedicated fax DID configured on your FPBX fax module. I could run some network packet capture tool if you have SSH access to the Asterisk server, and pin-point why the call is dropping ( and find the appropriate resolution ). You may contact me at any time for more details.
Thank you.