Issue with team call groups and Verizon callers over SIP trunks

[Robert Leist]

For anyone that has SIP trunks and users that utilize team call groups, have you experienced any issues with calls from Verizon callers?

Team call groups can support up to 20 Skype for Business endpoints to simultaneous ring calls to. All of those endpoints generate 183 Session progress and 180 Ringing SIP messages when they are being simultaneous rung. Skype relays all these 18x messages to the SIP trunk that the call came in on. As our SIP trunk provider has been setting up SIP peering with other carriers, all these 18x messages are getting relayed to the other carriers.

I don't know Verizon's threshold, but these multiple 18x messages are causing Verizon to mark these calls as denial of service attacks and dropping the call on their end.

Is there any way to suppress the multitude of 18x messages from team call groups at the mediation server so they are not sent to the SIP trunk?

We have tried manipulation rules in our SBC that indicate our SIP trunk doesn't support multiple 18x messages. This works initially for basic calls, but as calls are transferred, the original caller doesn't get additional ring back as additional 180 ringing messages are being suppressed by the SBC.

Thanks

[#2]

Just wanted to add that we started experiencing the issue a few months ago and it appears to be specific to Verizon. It didn't make sense because we have different size groups and more and more were reporting the problem, so it didn't seem to be the 20 endpoint cap on Team Call groups.

Never occurred to us that something internal was being rejected by a carrier. This will help us restructure to mitigate the issue.