I have a 2860 with an ADSL and two USB 4G modem WAN connections on different carriers. The USB sticks are connected using DHCP, and I assume are behind CGNAT. The router is set up to do session-based load balancing across all WAN connections.

After adding the second USB connection, my VoIP phone SIP registrations failed. I added a route policy to put UDP 5060 over the ADSL connection only and that fixed it.

1. Please would someone explain what is going on here?

2. Is the route policy is enough or should I be doing something else to properly address this issue?

Perhaps the concept of a 'session' isn't applicable to UDP traffic?
(Though that wouldn't really explain why it wasn't a problem with ADSL and one USB stick...)

I wrote:
(Though that wouldn't really explain why it wasn't a problem with ADSL and one USB stick...)

Unless...

The ADSL and first 4G connection had some disparity that meant that Voip/IP traffic always used the same one. Perhaps the introduction of the 2nd 4G connection, means there are now two equally valid paths that could be used?

I've no idea how the Router decides to load-balance. With my (admittedly very limited) experience of trying it with the 2860, theoretical link speed seems to be a big part of it...it certainly prefers WAN2 over the VDSL, even if WAN2 goes nowhere useful (and you've not even enabled load-balancing )

Thanks very much for the replies @hornbyp.

It's certainly puzzling. Maybe I need to read more about VoIP protocols to make things clearer. As you say, it's curious that it was the introduction of the second 4G connection that broke things. If CGNAT were the cause of the problem, I would have thought it would break with the first 4G connection.

I also don't understand why the RTP works. Maybe establishing the SIP connection forces it to use the same IP and hence WAN interface.