ZyXEL Communications U-336SA - Manual (Page 76)

62 Special Functions
2. Set S51.0=1 to enable EDR and report RING twice. The modem
will not report a normal RING and ZFAX will not answer a call
unless EDR RING is reported.
3. Set S51.4=1 to enable fax CNG tone detection. It is reported as
RING.
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.