Over the past year or so, I've had a dozen or so calls that have been dropped for no apparent reason. I just chalked it up to "the internet." However, today it happened twice in a row, and so I checked my logs.
I found that, in each case, Asterisk had detected a fax and attempted to transfer the call to the Fax Extension (which doesn't exist). I dug further and found that FreePBX includes faxdetect=yes in sip_general_additional.conf, and affords no way to disable it.
I don't see why this appears in sip_general_additional.conf. And it seems to me that it should not for the following reasons:
1. Most people who use FreePBX don't use the fax capability at all.
2. faxdetect=yes can be included in the trunk settings for trunks that need that feature.
3. If faxdetect=yes is going to be included in the [general] section, then there should be a corresponding option in the Asterisk SIP Settings module or the Fax Module to turn it off.