All, I ran into this issue myself for the first time today. I have 3 SSIDs 1 private and 2 guests. Guest uses NAT mode. I read all the posts on this thread and (1) kind of worked. Under Firewall & traffic shaping, I set LOCAL LAN access from Block to allow, and I created ACLs to block 172.16.0.0/12, 10.0.0.0/8 and 192.168.0.0/16. After I did that, the problem became sporadic. The guest users could make a call, and about 10 to 15 seconds later the call would go silent on both sides, and sometimes the call just wouldn't connect just like when the problem first started. Here was my fix, that worked for all my sites.
I kept the ACL rules I described above, but instead of letting the AP proxy the DNS requests, I changed Content Filtering from "Don't Filter Content" to Custom DNS, and I issue the Guest 8.8.8.8 and 1.1.1.1.
It worked 100% of the time after that. I don't want to pretend I understand the programing here, but I think that when "Don't Filter Content" is enabled, the AP takes the users DNS requests and makes the request on behalf of the user. I think something gets lost in translation when it comes to wifi calling, and wifi calling breaks. Maybe the @Meraki support team can do something with this information to fix the problem, but this 100% worked for me. I hope it works for you...Good luck out there my friends!