At several of our offices with Meraki MX firewalls we have secondary internet through a cellular provider using an ethernet connection to their gateway device. We noticed that when WAN1 failed and WAN2 on the cellular gateway took over we would have unstable/intermittent issues with the site-to-site VPN. Meraki support determined that packets were being fragmented and that was the source of the issue. Only they can set MTU and they changed it to 1480 for the WAN interfaces which corrected that specific issue. I was told it had to be set on both WAN interfaces and could not be set on only one. A new problem we noticed as a result of that is that our Yealink VOIP phones using Zoom Phone cannot make outbound calls. They can receive just fine, but outbound calls just time out. Using the Zoom app on a computer or mobile device works fine, it's only on the desk phones. Using a flow preference for the voice subnet to use WAN2 works around the issue. I've not been able to find a way to set MTU on the Yealink phones to test the result of setting them lower but I'm confused by the inconsistent behavior on the two WAN uplinks. I'm honestly not comprehending what's going wrong and would appreciate if anyone can shed light it. I'm happy to read any kind of documentation, blogs, etc. but nothing I've read so far has helped me wrap my head around the dysfunction happening.
Thanks!
