So, to expand upon this a bit more, there's a few steps I'll personally take if I suspect LAN issues.
First, I'll check to make sure that port 7000 is being used or not. If not, I'll switch the phone to using port 7000 as 5060 is very commonly caught by router's SIP ALG and as we all know, SIP ALG is universally TERRIBLE.
If it persists after that, I'll enable TCP transport instead of UDP. There's some overhead that comes along with this, but it's not too terrible and it can resolve issues like UDP fragmentation.
If it STILL persists after this, I'll try TLS. It's rare, but it is still possible for routers to determine that traffic on port 7000 is SIP traffic using deep packet analysis. TLS necessarily means TCP is being used. So it's sort of the nuclear option to make sure that there aren't any network config issue.
If there's STILL a problem, I'll break out a network monitoring tool like PingPlotter to monitor the connection for packet loss/jitter. There are a few other tools that'll do this well too like Solarwinds NPM, MTR and a cool secret tool we are testing right now. (stay tuned on that one)
There has been only ONE time that this procedure hasn't resolved the clients' issue without there being some major issue that was affecting large swaths of clients on a cluster. And that was a ticket from @Karl Stallknecht. I honestly can't remember what came of it though. Maybe he'll share? Think maybe it was an edge case in the code?