Situations like this point to the voice quality provided by your network infrastructure not a CODEC compatility issue or even CODEC performance issue. To troubleshoot this problem you will first need to determine the scope of impact. Meaning, what areas of your network need to be involved for the quality to become poor.
1. Check a phone call between two phones on the same LAN network switch 2. Check a phone call between two phones on different LAN network switches (you may need to check other switches) 3. Check a phone call between two phones on different sites attached via a WAN 4. etc.
5. Check calls to/from the PSTN
You may have to try different scenarios to start getting an idea of scope. In this exercise you aren't really focused on finding what exact scenario causes an issue as much as you are excluding scenarios that do not cause an issue.
Once you find the scenario(s) that demonstrate the issue, you will then need to check if there are other factors such as call duration, who places the call (e.g. PhoneA has a problem only when a PSTN caller calls PhoneA, outbound calls are OK), call features used during call, etc.. Using this additional pieces of data you can start honing in on root cause.
You may also ask yourself questions like:
1. Is it all phones or just a subset of phones that are affected? For example, if it is always the same phone that has a problem, but all other phones are fine, then you have found your culprit.
2. Are phones of a particular model the only ones reporting the issue? If so, then you have to look at firmware levels.
3. Are phones having the issue ONLY when calling (or receiving calls) the PSTN? Then you need to look at the DSPs on your voice gateway. You need to determine DSP firmware, IOS version, etc. and check for defects using the bug toolkit.
4. Are phones having an issue calling other IP phones when phones are on different switches? You could like at design things:
- QoS configurations
- Are voice and data on separate VLANs as they are supposed to be
- How large is the voice IP subnetwork? Is it larger than 512 hosts. If so, this is a violation of Cisco best practice.
- Are inter-switch connections presenting a choke point due to not enough bandwidth. Proper qos configs should mitigate the impact of this unless the problem is extreme and you would likely not need to ask what your issue is as it would be obvious.
You could look at operational things:
- QoS performance (see if packets are dropped)
- Link performance. Particularly on uplinks, you will want to check for errors on the interfaces. I have had many voice quality issues be resolved by swapping out a bad GBIC or compromised fiber connection. If you see errors on your uplink interfaces. Resolve them before checking/testing anything else.
There are plenty of things to check, consider this a jump start. Summary: Isolate your issue to something that is better defined by excluding scenarios that are OK. Drill into the various permutations of the remaining scenarios for further exclusion (and possible root cause identification). Check design, check configurations, and check interface/DSP statistics.
HTH.
Regards,
Bill
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