Switching to VoIP

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Chapter 15: What Can Go Wrong?

used by the SIP registrar to send its 200 or 400 response, depending upon whether or

not the registration attempt was successful. Since private addresses aren’t routable

on the public Internet, the response never gets returned to the registering endpoint,

which interprets a timeout. Registration fails and the SIP endpoint can’t make any

phone calls.

The solution to this problem is also to use a SIP proxy that runs parallel with the

NAT firewall. But the trick is that the SIP proxy must have a public address so that

the remote SIP registrar’s responses have somewhere to go.

The IP phone can’t make any calls

Assuming the phone’s IP configuration is correct, this problem normally occurs

when the phone is unable to register or log on to the SIP registrar or H.323 gate-

keeper server. Without an outlet to place calls, which is what these servers provide,

the IP phone is unable to call anybody.

In SIP setups, this condition results in a 403 (and sometimes 401) SIP response, indi-

cating registration failed. If the SIP setup uses a proxy, then response 407 occurs.

Either way, the authentication of the IP phone has failed, so it can’t place calls.

(Chapter 11 demonstrated a failed SIP registration using Ethereal.)

To resolve the issue, be certain the phone’s ...