Hi all,

We recently upgraded our Mikrotik to RouterOS 6.41.2, right after the upgrade a SIP server with a public IP stopped being able to transmit fax over ip, the router does bgp and is the dfw to to public IP of the SIP server, so there isn’t really any NAT going on, but since this only started right after the upgrade of RouterOS, which did solve other issues we were having, now we’re stuck with this fax over IP issue.
Can anyone provide some assistance or where to look?

Thank you!

As you say you do not need any NAT, can you try (and, by some sources, reboot the router before testing whether it has helped)? The thing is that the SIP helper may not like something in your T.38 SDPs, making it not to forward the messages carrying it. I had a similar experience long ago (not with T.38 but with an item in SDP which the Mikrotik did not like).

If that does not help, sniff the traffic at both involved interfaces of the Mikrotik into a single file and inspect the file using Wireshark to see what exactly does not pass through the Mikrotik (or passes through malformed).