This is most often caused by the Expiry Time setting in your VoIP device being shorter than the Expiry time used by your VoIP provider’s SIP server.

Background:  Unlike your Telstra PSTN line, a VoIP connection may not always be connected – for example someone who travels while using a softphone on their laptop computer.  When travelling, the laptop is offline so their VoIP softphone is not able to receive calls, and its IP Address changes at each location whenever it reconnects to the Internet.

To allow for these mobile users, the VoIP provider’s SIP server only keeps your VoIP device’s record active for a certain time.  After this time any incoming calls will not be forwarded to your device.  Your VoIP device should send another registration request to the SIP server before the SIP server cancels the previous registration.

Symptoms: If your Expiry Time setting is longer than the SIP server’s setting, you will find that after registering, your device can make calls until the SIP server expires your registration, then you will not hear a dial tone until your VoIP device registers again.

Solution: Reduce the Expiry Time setting in your VoIP device’s SIP Account profile.  The value required will depend on your VoIP provider – for example SPANtalk uses 30 minute expiry, but MyNetFone uses 4 minute expiry.

Note that setting your device’s Expiry Time to a value lower than the VoIP Provider’s expiry time ensures that your VoIP device stays registered.  The only disadvantage is using a much lower Expiry Time will result in more registration messages than are really necessary.