Value added faxing through SIP trunks


Hello all,
I am having issues getting the Value-Added Features - Fax Sending tool to send a fax over a SIP trunk. I can receive them just fine. When trying to send the fax out it does not seem to start the Auto Negotiation that you normally hear within a fax. Has anyone figured out how to get this to work? If so any help would be appreciated.


As it’s over VoIP, you won’t hear anything…


Would there be another reason the fax would not send?


It depends also on your SIP trunk provider. They have to support T.38 faxing. Not all of them do.


I am lucky, The provider is a sister company. They do support T38 faxing


First, check that you are running the latest firmware. This feature is known to work and break depending on firmware version.

Then, check that you are using the good configuration for the provider. Meaning: Remote-Party-ID if needed, etc.


I am running the latest firmware and also have a good configuration with the SIP trunk. The system can make and receive calls, it can receive a fax that is going to an email. It wont send a fax out tho.


In this case, please open a ticket with Grandstream at

They will be able to help you further. They will need logs and such.


And by what means sending? Value add, email to fax or fxs?


Value-Added Fax Sending feature.


Thanks fmarcoux96, I should have seen that.

Do a network capture of a fax call and post the pcap file. There can be any number of issues that cause this. RPID is specifically excluded by some providers as it implies a remote fax relay such as might be encountered from a call that is forwarded. I know of several that will pass without RPID, but will return a 488 if RPID is sent as I assume they compare a request URI to the RPID.

Also, while your provider may support t.38, does not necessarily mean that the entire path along the way to the destination does and it could be that the issue is upstream from your provider and it could be that the attempt to send is done using g711u (for your locale) and the packet switch network along the way is unable to support the demands that fax requires.

You might also open ports 4000-5000UDP and forward them to the UCM.