sip.conf:

t38udptlsupport=yes

[fax]
type=friend
secret=fax
host=dynamic
qualify=yes
dtmfmode=inband
disallow=all
allow=ulaw
context=faxtest



extensions.conf:

[faxtest]
exten => 1234567,1,RxFAX(/tmp/fax.tif|caller|debug);



fax.call:

Channel: SIP/fax
MaxRetries: 0
WaitTime: 60
Application: TxFAX
Data: /var/tmp/test3_opbx_rulez.tif|caller|debug