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Cisco IP Telephony Troubleshooting Guide for Cisco CallManager Release 3.0(1)

levels (start with 50% of the maximum audio level). Most of the time, the problems will occur when attaching to the PSTN by way of a digital or analog gateway. Cisco IP Phone users may complain that they hear their own voice being reflected back to them. Now, although the true source of the problem is almost always at the far end, it is nearly always impossible to change anything in the PSTN. So the first step is to determine which gateway is being used. If a digital gateway is in use, then it may be possible to add additional padding in the transmit direction (towards the PSTN), in the hopes that the lower signal strength will yield less reflected energy. Additionally, you can adjust the receive level so that any reflected audio is reduced even further. It is very important to remember to make small adjustments at a time. Too much attenuation of the signal will make the audio impossible to hear on both sides. Alternatively, you can contact the carrier and request to have the lines checked. On a typical T1/PRI circuit in North America, the input signal should be –15 dB. If the signal level is much higher (-5 dB, for example), then echo will be the likely result.

A log should be kept of all calls that experience echo. The time of the problem, the source phone number, and the number called should all be recorded. Gateways have a fixed time of 16 ms of echo cancellation. If the delay in the reflected audio is longer than this, the echo chancellor will be unable to work properly. This should not be an issue for local calls, and long distance calls should have external echo chancellors built into the network at the Central Office. This is one of the reasons why it is important to note the external phone number of a call that experiences echo.

Gateway and phone loads should be verified. Check CCO (Cisco Connection Online at www.cisco.com) for the latest software loads, new patches, or release notes relating to the problem.

One-Way Audio or No Audio

One-way audio occurs when one person cannot hear another person during a call. This can be caused by an improperly configured Cisco IOS Gateway, a firewall, or a routing or default gateway problem, among other things.

There are a number of causes for one-way audio or no audio during a call. The most common cause is an improperly configured device. For instance, Cisco CallManager handles the call setup for a Cisco IP Phone. The actual audio stream occurs between the two Cisco IP Phones (or between the Cisco IP Phone and a gateway). So it is entirely possible that the Cisco CallManager is able to signal to a destination phone (making it ring) when the phone originating the call does not have an IP route to the destination phone. A common cause for this is when the default gateway in the phone is improperly configured manually or on the DHCP server.

If a call consistently has one-way audio, take a PC that is on the same subnet as the phone and has the same default gateway and try to ping the destination Cisco IP Phone. Then take a PC that is on the same subnet as the destination phone (with the same default gateway as the destination phone) and ping the source phone. Both of those tests should work. Other things can affect the audio traffic include a firewall or packet filter (such as access lists on a router) that may be blocking the audio in one or both directions. If the one-way audio occurs only through a voice-