We had a couple reports of ringback issues with Verizon handsets last week. The SIP signaling doesn't have any oddities like you experienced but the customer indicated that they heard dead air instead of ringback tone. Your signaling looks like Comcast sent it out provider #1 which sent back a 183 ringing but a time out timer expired and they route advanced to a second provider which sent back a 180. If the time that you received the first 183 packet was small, then this theory is dead. On Fri, Oct 29, 2021 at 12:11 PM Hunter Fuller via VoiceOps < voiceops at voiceops.org> wrote:

Is anyone else getting an eerie lack of ringback when originating toward T-Mobile?

We get a 183 Session Progress with media, then a 180 Ringing a third of a second later. So the result is a 1/3 second long burst of ringback and then silence. We are seeing if we might be able to start locally generating ringback when we see the 180, but to be honest, that's just a weird sequence of messages and I would never expect to receive them in that order. So we didn't exactly plan for it.

The relevant trunk is from Comcast, but I highly suspect they are just passing through what they got from T-Mobile (or whoever they route T-Mobile calls through). We are in the north Alabama area and we have heard of other customers with various carriers having similar issues.

Just wondering if anyone had a technical reason/explanation or any more information.

-- Hunter Fuller (they) Router Jockey VBH M-1A +1 256 824 5331

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