Taking a closer look at what you reported here are some considerations., here are a few potential causes and solutions for high jitter:
Network Congestion: This is one of the most common causes of jitter. If your network is congested with too much traffic, it can lead to increased latency and jitter. You mentioned that network usage is not particularly high at the moment, but it might be worth checking if there are specific times or conditions under which the jitter increases.
Hardware Issues: Old or outdated hardware can also cause jitter. Even though you mentioned that all users have the same wifi adapter and driver version, it might be worth checking if there are any hardware issues specific to the affected users' devices.
Internet Connection: A weak or unreliable internet connection can cause jitter. Upgrading your internet connection or using a wired connection instead of Wi-Fi can help reduce jitter.
Packet Prioritization: If voice packets aren't prioritized, then the end user is very likely to experience jitter Implementing Quality of Service (QoS) settings that prioritize VoIP traffic can help reduce jitter.
Bandwidth Usage: High bandwidth usage can also cause jitter. Try to minimize the usage of bandwidth-intensive applications during VoIP calls.
If none of these solutions work, it might be worth reaching out to your VoIP provider or a network specialist for further assistance. They can help identify any specific issues with your network configuration or VoIP setup that might be causing the high jitter.
I also suggest you open a support case.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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