Typically the default settings will work well for faxing, but if you are having issues the below options have been known to help in those cases. If faxing issues continue after these options have been applied then contact Sangoma Support.
set .media.pkt_profile.1.fax.tx_level=-12
set ._advanced.media.control_V25=fax
set ._advanced.t38.allow_ecm=1
apply
save