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Troubleshooting T-1 PRI Trunk AudioCodes Configuration: Microsoft Lync and AT&T B VoIP T-1 Router

I ran into a T-1 issue with a customer T-1 circuit this last weekend that I thought might be helpful to others in troubleshooting their circuits.


I was troubleshooting from an AudioCodes Mediant 800B, that being the voice gateway for a Microsoft Lync 2013 VoIP solution. It should be noted that the version of Lync doesn’t matter as this was an upgrade from 2008, 2010 to Lync 2013.

The provider connection is a T-1 from an At&T IP Flexserve router offering the B-VoIP service. This is only important because the T-1 coming from the AT&T router, isn’t the same T-1 as a normal T-1 (copper in the ground) through a wire-center to your business.

Issue or Problem

The T-1 and corresponding voice service was up and running for at least 3 years. There was a circuit issue from AT&T where they took down the wrong circuit, dropping the T-1 connection on the router. From the Audiocodes Mediant 800B, we just saw the circuit/port go down.

Troubleshooting Steps:

Before I was pulled in, the customer reset all equipment, the AT&T FlexServe router and the Mediant 800b voice gateway. Neither of these had any effect on the down T-1.

When I first got on the voice gateway and looked at the PRI, I saw it was in AIS alarm and Channel 24 was the only one having issues. I tried some minor changes on the port, but nothing seemed to bring the T-1 up. I spent 2 ½ hours on support calls with AT&T. I had a real issue getting their support to perform any type of troubleshooting as they kept saying that I needed a cross-over T-1 cable. Since I was troubleshooting remote, I was unable to validate the T-1 cable. I still wanted to validate that all other parameters were checked. The AT&T tier 2 support tech just dropped my call.

After calling back and wasting another 45 minutes on hold and in queue. I was finally able to get AT&T to send a loopback across the T-1. I could not see the remote loopback. When I pushed a loopback from the Mediant 800b, the AT&T tech could see my loopback, or at least that is what he said. Still couldn’t get the circuit up.

The next day I could get a tech onsite to verify that the cable connecting the devices was a cross-over T-1 cable. It was. But the cable was only 3 feet long. The minimum cable length is typically around 100 feet.

Not wanting to hang 100 feet of cable off the rack, I was able to change the “Line Build Out Loss” from 0 DB to -22.5 DB. As soon as I made that change and reactivated the circuit the PRI came right up. My thought was AT&T had added this configuration to their side. When they rebuilt the T-1 configuration they used the standard defaults, thus requiring our side to add the configuration.

So in troubleshooting PRI or T-1s, if your cable is correct (cross-over cable), don’t forget to look at your cable length or try adding some DB loss on the line as a standard troubleshooting step.

Jason Howe, PEI


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