HT 802 Fax not going through properly


We have a new HT802 and we are using it for outbound faxing in conjunction with a SIP trunk which is T38 certified and customer’s physical fax machine. When trying to send a fax, we hear dial tone, can dial the external fax number, we hear fax tone but somehow when fax is being sent to a fax machine which is connected to a POT analog fax line, the handshake doesn’t get completed and the fax doesn’t go through. We get a failed fax notification. We have tried several things suggested in this forum but somehow no success yet. Any suggestions?


It is time for basic troubleshooting.

Does the receiving fax machine get any portion of the document?
Does ;your fax machine have problems sending to other fax machines?
Does the receiving fax machine have problems receiving from other fax machines?
Can you put a telephone in place of both fax machines, then do a voice call to listen for problems?
These questions might indicate where to look for a problem. Are SIP packets being lost? Is the POT line having interference or poor connection?

My suggestion is to simplify as much as possible, and if that works then add complexity back in one step at a time. Set your sending fax machine transmission speed to be for a VOIP line (if you don’t have that setting then set to the slowest speed).

The handshake process includes testing the telephone call connection quality to find out what transmission speed can be used. If the failure is truly happening in the handshake then you have a poor quality call.



Start by doing as RIckL suggested by setting the speed lower. Does the receiving fax machine have issues receiving/sending faxes from/to other landline based fax machines (preferably more than one site)?

Now for the bad news. FoIP (Fax over IP) is never a sure thing. Simply because your ITSP offers t.38 only means that it supports the protocol which improves the odds of a fax being successful. Unfortunately, your ITSP is probably not the end-to-end carrier between your origination point and the fax destination and the transmission stream may need to wind its way between numerous carriers along its path to the receiving fax. In doing so, the data stream as it left the HT is often not the same in appearance when it arrives. The t.38 protocol strives to overcome some of these issues, but it is not superman and can only do so much. What it can’t handle results in a failed fax.

Fax will only use t.38 or a g711 codec. In the HT, set all the available codecs to g711u/PCMU. Additionally, set the HT such that it will only accept SIP messaging from the SIP Server/Proxy. When sending a fax, be aware of the fact that the longer the fax (number of pages) the higher the stakes become in favor of a failure.

Insure that the network to which the HT is connected has the needed bandwidth and priority (QoS) to be able to send a fax so that its data stream is not interrupted or prevented from reaching the Internet due to congestion on the network or from your Internet connection. Keep in mind that as those data bits leave the WAN side of the router on their way to wherever, you no longer have any control what happens thereafter. This is why slowing the speed when sending may give the receiving side a better chance.

Hopefully, you have other locations by which to test and are doing so.


I found this guide helpful.