Ringcentral not making calls

TL;DR: Reduce the MTU on your laptop to enable Ringcentral to make calls.

The other day I was testing RingCentral on my laptop to replace desktop phones. The mobile app worked perfectly, but the desktop app would not make outbound calls. They were “connecting” but never rang the destination phone.

A few PCAPs later and I noticed the router was sending ICMP messages about not being able to fragment the packets being sent. Reducing the MTU to eliminate the fragmenting problem, and everything worked as expected.

The windows commands that were run are:

netsh interface ipv4 show subinterface
netsh interface ipv4 set subinterface interface=Ethernet mtu=1400 store=persistent

I hope this helps.