A customer is having intermittent 1 to 3 second audio delays on external voice calls. Internal calls seem to be fine. The issue is happening on both inbound and outbound calls. Total call volume is ~50 calls per day and about 25% to 30% of the calls are impacted. When I call their numbers from my cell phone, I can not recreate the issue. This installation was done about 3 years ago and was running without issue until 2 to 3 months ago.
The 3CX is cloud hosted (version 20.0.8.1121) with a 4 channel SIP trunk from siptrunk.com. There is a local Windows server that has the 3CX SBC (version 20.0.100) on it, the server has very low usage. There are 14 phones at the site, which are all registered to the SBC. The internet connection is 180 Mbps both ways and the usage is low. End users are all on Yealink T46U phones running the latest 3CX approved firmware.
I have turned on the connection quality monitor for all users and logged calls for 1 week. I had the customer record their experience with each call and matched their logs up with the connection quality monitor. For all calls, even the ones where the users reported audio delays, the round trip times are in the 50 to 80 ms range, the jitter is ranging from 3.5 to 7 ms, and the score is excellent for audio to/from the 3CX to the phone.
I need help figuring out my next troubleshooting steps. The 3CX is telling me the calls are good, siptrunk.com support is saying they don't see any issues on their side, but the users are experiencing frequent audio delays.