I've seen this with the prev Elastix Distro. After a fair amount of faxing I'm seeing some of the IAX2 extensions become permanently busy when dialed locally with 3 digit or directly to the 10digit DID. If I delete and rebuild the IAX2 fax and extension it seems to move on to another IAX2 fax extension. I'm able to reproduce and captured an Asterisk -r of this happening. Since I can reproduce with 3digit dialing I know it's not the SIP trunk I use. Since the problem happens on other extensions at random I assume that it may be an initialization problem. Please see debug attachment conveniently DIFF'ed side-by-side with a working and non-working call. The problem seems to be at "Everyone is busy/congested at this time (1:0/0/1)"