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Cisco 7912

Posted on 2014-01-21
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Last Modified: 2014-01-29
we have multiple users saying that their 7912 phones have static on the line, the caller cannot hear them or the phone call breaks up. we have the latest firmware installed. Cisco says to add QOS  to the config. We have added it and it will work on some devices and on others it will not.
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Question by:beltonisd
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Static problems are almost always network related.  QoS can help if properly set across all devices, but it is not the fix to underlying issues that may exist.  For example, a data switch with a type mismatch on the uplink NIC  or user NIC (half duplex versus full duplex) will cause static and dropped packets.  But there are many more potential sources.  The first thing you should do is run some tests to see if you are experiencing packet loss.
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by:agonza07
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Make sure your QoS is end to end, routers, switches, etc.

Not sure about the 7912s but on 7940s if you hit the ? button twice it'll give you QoS stats. Check out your jitter and other stats.
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by:José Méndez
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Sounds like these are PSTN calls, correct? What type of trunk is there to the telco? POTS? PRI? SIP Trunks?

Where is the border gateway located versus the ip phones? A different network? Maybe firewalls in between? WAN links? VPNs?

Helps us understand the situation.
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by:beltonisd
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The problem we have is not with all of our 7912's. We could have 2 of the same devices side by side, one works correctly and the other will have static. No VPN links. no firewalls between campuses.
We have PRI  trunks to telco.
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by:José Méndez
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Which side hears static? The outside or the inside user?
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by:agonza07
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Have you tried swapping out handsets and cords with a working unit?
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by:beltonisd
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we have static on the inside.
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by:beltonisd
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We have swapped out the phone cord and hand set first thing we tried. This is happening on multiple devices. Swapped with a different model and no static.
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by:José Méndez
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Would you be able to grab a packet capture from the back of the IP Phone?
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