Unity Connection - Some Voicemails Drop At Exactly 20 Seconds

[kylebrogers]

Setup -
Unity Connection 12.5SU8
Telco Connection - MGCP-controlled PRI (AT&T IP Flex, so PRI handoff to us, but SIP on the other side of AT&T's router)
SCCP CUCM/CUC integration

Some callers are cut off exactly 20 seconds into leaving a voicemail.
No issues with non-voicemail calls getting dropped.

Q.931 traces pulled from CUCM of those calls shows that, at the 20 second mark, we receive a DISCONNECT from the telco.
It's "random", though I'm beginning to think that it's caller/provider specific. These aren't callers we can call back and ask to help us do troubleshooting but in some cases the same caller will leave multiple 20-second voicemails so they can leave all of their necessary information. I've listened to a bunch of the voicemails and all of them are cut off mid-sentence.

Troubleshooting:
Multiple Unity Connection cluster reboots
G711 on all calls
NTP on both CUC servers, CUCM, and the gateway are all in sync
PRI is running clean (no slips, CRC errors, etc)
VAD is disabled in CUC
Trailing Silence is maxed out at 5 seconds (though I'm 100% certain this isn't the issue based on the voicemails I've listened to)
Upgrading from 12.5 SU3 to 12.5 SU8 (12.5.1.18901-2) to ensure that we're not hitting bug IDs CSCvm68778 or CSCwc37421

The issue is still occurring.

[Bill Talley]

Hi Kyle,

Curious if you've made any progress with this issue?  I have two customers who started having the same issue about 6 months ago, also on AT&T IP Flex PRIs.  TAC has pushed it off as a carrier issue because of the inbound disconnect from AT&T like you're experiencing. 

One customer migrated to new PRIs, still IP Flex, but different telco hardware and the issue persists.   The second customer migrated to IP Flex sip trunking and the issue went away.   

We've tweaked all kinds of timers on the gateway with no success.  We have other customers with PRI handoffs from other carriers who are not experiencing the issue so it's definitely a unique issue, seemingly with AT&T PRIs.

TIA,
Bill

[kylebrogers]

We haven't made any progress yet.  Getting a knowledgeable layer 3 tech at AT&T on the phone who can properly trace calls back through their equipment is like trying to nail Jello to the wall.  Just an endless round of "we're sending out a circuit tech" followed by "we're escalating the issue" (when everyone is getting "escalated", nobody is).  I actually managed to get one on the phone for a different customer/issue recently and it took the guy maybe 30 minutes to look at some calls and give us an answer to a question we'd been asking for weeks.  It's just getting them on the phone in the first place that's difficult.  Interestingly, that issue (again, completely different issue) only existed when the outbound call was routed to someone on AT&T mobile from our AT&T IP Flex PRI.  Outbound calls to other carriers had no issues.