ZyXEL Communications U-336R/RE User Manual page 63

4. Set S51.2=1 to enable DTMF tone detection. It is reported as
RING. If the remote fax machine does not generate the CNG
tone, ask a fax caller through your answering machine message
to press "*" (or any other DTMF key) if he or she wants to
send a fax.
Save the settings in a profile and reset the modem with AT&WZ.
Special Functions 51