Help with a weird network setup and a UCM6202


Hello everyone,

I have a network setup that the topology is like this:

Fritz!Box 7590 > USG-3P > Mikrotik Router

The Fritz!Box is on the local network The USG-3P is connected to the Fritz!Box through the WAN port using DHCP. After that the USG gives addresses on the range of from the LAN port. On the LAN port there is a Mikrotik router that basically works on the range of and every device that I connect on it gets address from that range. What I am trying to do is connect a UCM6202 and a DP752 (assigned on it are 3 DECT phones) on the Mikrotik and make it work from there. Let me clarify a few things: The Fritz!Box has the telephony lines from my ISP and I have created two VOIP trunks on the UCM6202 to make it work on the PBX. When the UCM, DP752 and the Fritz!Box are on the same subnet everything works flawlessly. But when the UCM & DP752 are on the subnet I get no signalling when I call, no audio on outgoing calls and no incoming calls.

I am really sorry if this is a very elementary/simple question but I am very new to these things, I have spent countless hours to figure out what is going. I also tried to do port forwarding on the USG with no luck (I do not know if I did it correctly however).

Every help is much appreciated!

Thank you for taking the time to read my post.

EDIT: I have disabled ALG on the USG using config.preperties file.


you have to nat everything going out the mikrotik to to the usg

so src ip to dst masquerade /nat if you not sure how to do this ask on the mikrotik forum.


Hi telecomsolutions!

Thank you so much for your reply.

I did that. But no luck.

Here is an interesting fact. When I connect the UCM directly to the USG I have the exact same problem. Maybe I need to NAT from the USG to the Fritzbox?


Does fritz box not have a built in router where is it getting its Ip from


The fritzBox IP address is on the subnet and yes it can work as router as well. Here is another interesting thing that i have noticed: There are no SIP packets when I use as capture filter the following:

Isn’t that weird? I mean it should at least show something when I use “host” since I am on the same subnet…


I finally got some packets captured from wireshark and these are mostly tcp and tls packets.


3 x Routers - which one or one(s) provide the internet? why do you need the USG ? or even the Mikrotik ?

Wouldnt you just need the Fritzbox and use the routing tables to do what you want with that?

Where from? the ISP?

This would indicate that the Mikrotik and USG are not needed, unless you have another reason for using them outside of them being there to use.


Hello scottsip!
Thank you for your reply.

Well I have other devices on the network from Ubiquiti with a controller etc and I need it. Plus I have enabled IPS/IDS. Now the mikrotik basically works as a switch and since it has 10 ports which I use 8 of them I am unable to remove it. The Fritz gets the telephony from the ISP I just created 2 telephone devices (that is the way fritz calls it. Basically these are just 2 VOIP devices which they have usernames and passwords that I define and after that I created 2 VOIP trunks on the UCM which use the same credentials as the 2 VOIP devices and that is the way I bridge the telephony from the fritz to the UCM. Unless there is another more effective way to do it and I am not aware of it.)

EDIT: The fritz provides the internet.


Then you need to do the following of 2 ways…

Internet --> Fritzbox --Mikrotik (not as a switch) --> network (Pcs and UCM)

The USG is not needed in that or

Internet --> Mikrotik (not as a switch) in Router mode --> network (Pcs and UCM)

Mikrotik Provides IPS/IDS


OK. I will try your network topology and I will post back if I have any issues.

Thanks again.