I would direct you to a ticket I opened, #772027
Please listen to the 16 call examples and relevant portions of the conversations I provided across two completely different sites. The only common denominator is both sites are using S500s. The problem can be easily reproduced and it only took about 4 hours of calls at these two fairly low-volume sites (only recording a handful of extensions) to provide these examples. External calls, internal calls, all share the same issue. The LANs are flat, small, and pretty basic.
This problem is not a figment of my imagination. These sites are literally 100% Sangoma!
I just deployed a new site with Yealinks, on FreePBX 14. If there were any problem this particular client would be screaming from the mountaintops and excoriating me… they are that type of client. They do not share the same issues as the above two although they share a very similar infrastructure and same SIP provider to the other sites with S500 phones.