Dial tone lost when DNS servers down

I was unable to use my asterisk box, even for internal calls, when its DNS servers were down. Why would that be? I can understand why I wouldn’t be able to connect to a VoIP provider. But why would my local extensions/phones have dead air when I picked up the receiver? I was not able to dial out using the PSTN.

I am using Asterisk 1.2 and two analog phones plugged into a Sipura 2000.

Is the Asterisk server configured for the same DNS as your Sipura endpoints? Dialtone is generated ‘locally’ by the ATA, so it would appear that the Sipura could not reach and register with the Asterisk server due to the DNS problem. One way to change this behavior would be to configure the Sipuras to allow calls without registering, which may be configured in the Web GUI of the Sipura ATA. Then, even if they can not register with Asterisk, you will still get dialtone. The issue of course, is that if the DNS is down the Sipura would still not be able to resolve the SIP URI, therefore you would also have to use the IP address for your proxy.