FreePBX 13.0.120 does not allow changes to updtl.conf but also does not allow settings to be modified in the GUI. Do I need to set the values in updtl_custom.conf or are the defaults enabled on the system already? Anything to do in Asterisk SIP settings?

e.g. SIP settings:
T38 Pass-Through = Yes


Add to the custom file. After ‘core reload’, confirm settings at the Asterisk CLI:

*CLI> udptl show config
UDPTL Global options
udptlstart:      4000
udptlend:        4999
udptlfecentries: 0
udptlfecspan:    0
use_even_ports:  No
udptlchecksums: Yes
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That seems to be the default without adding anything to the custom file but faxing is pretty much hit or miss with these settings. I even have FaxPro installed (don’t know if that makes a difference)

SIP settings
T38 Pass-Through = yes

Fax config:
ECM = yes
min speed 9600
Max 14400

Are there any other values that need to be set in the GUI or conf files?

In my experience it doesn’t. Faxing is just hard to do once you get away from a carrier-class line.