FYI, I just ran into this issue on a few of our sites that were upgraded to 13.28, with the MX84 platform. There is a bug in this version that can cause very high CPU utilization, impacting latency sensitive traffic such as VoIP. The symptoms are very bad call quality, and increased RTT for IP traffic. From what I can tell, it seems to impact sites that have Cellular modems that may not have fully initialized, and are unable to connect to their respective telcos. Rebooting the device solves the issue immediately, but then eventually it does come back. Meraki's suggested fix is to either unplug the modem entirely, or revert back to a previous software version. I've reverted one of our sites back to 12.26. I should know by tomorrow how it will look.
However, there is one site where I had the user unplug, and plug the cell modem back into the MX84, and the issue hasn't appeared since, and it is still running 13.28.
Bug has not been fixed yet, support advised me they would send a notification when it has been squashed and what version to upgrade to. Only work around is to unplug the cell modem, or roll back to a previously known working version.