Upgrade Skype for Business Online (3PIP) phones to Microsoft Teams using SIP Gateway

Hello Pete, 

Thanks for sharing these details, much appreciated. Been facing  the same issue for quite some time now, it work intermediately on some users, on same model of different model; vvx 300, vvx400 and vvx500. All are giving same issue. 

I have brought AudioCode handset and found it work perfectly fine and it doesn't give any disconnection symptoms. 

Tried different firmware, 6.xxxx, 5.9, 5.8 It worked for sometime, after that when the calls droped it will trigger the errors; 

IceConnectivityChecksFailed
Q.850;cause=16
Forbidden
Temporarily Unavailable
IceConnectivityChecksFailed
Busy Here
Service Unavailable
EstablishmentTimeout
Server Internal Error

Logged a case with microsoft and they confirmed, its from their side; 

After consulting with our back-end team, this issue was identified as known issue for poly device. We have as well attached this case to the previously known issues already under investigation and forwarded your request to the engineering team to consider a one-time fix in the future regarding to call dropping. Our internal team advised that currently there are a lot of issue regarding to poly devices so our internal teams is working to fix those issue. Unfortunately, no ETA was provided. 

We will continue to keep an eye on this issue internally and work together with them on priority till the fix this issue. We will update you as soon as we receive any updates from the product team side. 
In the meantime, we will suggest user to use normal team's desktop apps as a workaround. 

Any workaround, as the business is heavily impacted, and ordering a new phones will take ages to arrive. 

Regards