Cisco SPA 303 Series Manuel de configuration et de dépannage - Page 9

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Phone.com Setup and Troubleshooting Guide

Cisco SPA303 IP Phone

and certain network settings or hardware might need to be

modified.

Here are some general observations and rules, based on our

experience. Consult the user guide for your network component, or

contact the manufacturer's support department if you need specific

guidance.

Make Sure You Don't Have A "Double NAT" Network

"Double NAT" (network address translation) is an issue where two

routers are placed one after another. This can be a common

scenario if, for example, your ISP provides a modem/router

combination device, but you've also added a wireless router of your

own. Although a computer behind a double NAT setup will usually

work fairly well, a VoIP phone will exhibit unusual behavior.

To resolve this issue, you'll need to put your modem/router device

into "bridged" mode; most will allow this. This will disable all routing

(NAT) functions on the modem/router, so only your standalone

router will perform these functions. Contact your ISP if you need

specific guidance.

You could also choose to purchase a standalone modem, to avoid

hassling with the ISP's hardware at all.

Disable the SIP Application Layer Gateway (ALG)

Originally designed to be a security component to help certain

types of traffic get through your router, most SIP ALGs actually just

make things worse.

Not all routers have SIP ALG enabled if they even have it to begin

with, but those that do interfere with VoIP traffic, often preventing

the traffic from reaching your phone correctly — or at all. We've

seen all of the following behavior:

• Your phone might not get a dial tone

• You could drop out in the middle of a call

• Your phone might not always ring on incoming calls

• The wrong phone might ring (if you have more than one)

• Your phone might continue to ring after being answered

Revised: May 28, 2014

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