Flash key not working on remote GXW4004 as analog line extender (with GWX4104)


Hi everyone!!
I have a customer that has extended analog extensions of a PBX with GXW4004/GWX4104 over a Wireless link.
I’ve followed the setup videos that are around there and everything worked just fine.
There are two issues reported however:
1.- When trying to transfer a call from the remote GXW4004, pressing the FLASH key does not work and comunication gets dropped. You should hear a dial tone after pressning the FLASH key to dial the new extension to transfer the call, but that is not happening. I wonder if this feature works with this kind of direct IP call setup.
2.- Customer apparently has a fax on one of these remote extension, and call gets connected but fax is not received…

What do you think? Any suggestion?
Thanks in advance for your feedback.


Hi Herman,

I would like to know if you found the solution for this issue.




you need to forget the Flash ® button on your Voip, otherwise you will run into continuous compatibility problems, line drop and various failures. Moreover, analog phones of different brands often use different times, a reliable and reference time is usually about 80 msec.
UCM has the codes *2 and #1 (or in any case those set) to make the trips with and without announcement.
Anyway you can try setting the ATA to a minimum time of 50 msec. and a maximum time of 100/120 msec. and make sure that the analog phones in use are about 80 msec.

As far as faxing is concerned, I have given up for a long time, too many variables, too many failures, no logic and a lot of time wasted :slight_smile:
Best wishes.


1a. Enable hook flash
1b. Set “Hook Flash Timing minimum” to either 80ms 90ms or 100ms
1c. Set "Hook Flash Timing maximum to either 81ms 91ms or 100ms
The value depends on the set value for flash in the analog phone.
Some phones default values (e.g. Gigaset) are set to 250ms e.g.
This is the value that fits if the phone is connected directly to the PSTN.
You shouldn´t use 250ms in the GWX or HT if it´s connected via UCM.
->instead set the flash value of the analog phone to either 80ms or usually 100ms.
Some phones do not show the milisecond value.Then mostly you can choose between “short” and “long”.Then you have to choose “short” at this point.
After you did this, you can play with the values of the GXV device to figure it out.
But as said before:100ms-min/101ms-max should score a goal.
2. Faxing is a PITA sometimes.
It depends on so many things:
1.quality of the internet connection
2.quality of the faxmachine itsself
3.quality of the faxmachine or service at the other side
4.settings of the faxmachine or service on the other side
5. gain sensitivity in/out of your faxmachine
6. jitter on your network
and so on and so on…
My settings that mostly work are those:
Gain TX:0db(default) or +2db
Gain RX:-12 or -10db
Line Echo Canceller:off
Network Echo Suppressor:off
Fax Mode:Pass through
Re-INVITE After Fax Tone Detected:enabled
Jitter Buffer Type:fixed
Jitter Buffer Length:high
codec:I have only PCMA (germany) the whole way from trunk to extension (you may need only PCMU!!!)
On analog Fax side: set Baud rate to 9600 and ECM to ON no matter what is in old documents that say ecm should be switched off ( yes, damiano70, I know you would set it to OFF :grinning: )
The latest version of the recommendations from the providers (at least the ones I use) is to switch ON electronic correction mode.
Also have a look that the dialtones match to your region (dialtone/ringbacktone/busytone)
If you do all these settings as above mentioned take a fax machine you trust (older ones mostly work better than the new ones)
Have also a look that SLIC Setting and Caller ID Scheme matches to your region/provider
If the fax machine doesn´t even ring, then you can switch to Enable High Ring Power
If all these settings are done, then you can play with the “gain settings TX/RX”
I would begin with the receive value RX.Start at -12db and do a testsending.
If it fails, the raise the value step by step and repeat the test.
If all values do not fit, then begin again with -12db and raise or lower the TX value one step
and begin the same procedure with RX value (raise it step by step).
If all fails then again raise or lower the TX value one step…and so on.
Matching the ratio of the send and receive volume so that it fits your device (receive and send tone not overdriven or underdriven) requires a lot of patience, sometimes you can find the right gain within 10 minutes, sometimes only after 3 hours and sometimes not at all. Sometimes it doesn´t work because the fax machine is no good on site or the network connection is crappy or the fax machine on the other side is messy. Combinations that still worked in the PSTN area can suddenly no longer negotiate anything sensible with VoIP.
Tip from my side: If you haven’t found the right settings after 3 hours, you won’t find them anymore.
I wish you success anyway!