Voxalot will update the registration the same second the ATA registers. Voxalot keeps registrations for 1 hour (3600 seconds). Ordinarily the ATA should register at 3600 second intervals, however if your public IP address has changed then you will not receive calls until the ATA registers again.
Before trying to solve the problem you should first determine if the problem is caused by a change in your public IP address. This is not likely, but if your IP address is changing frequently that would obviously cause you to miss some inbound calls.
More likely your missed call issue is because the router's ports are not open to the ATA at the time the call comes. If this is the problem the following settings should fix it.
Set 'NAT Keep Alive Intvl' to: 59
Set 'Register Expires' back to 3600.
Set 'NAT Keep Alive Enable' to: yes
Set 'NAT Keep Alive Msg' blank (yes, make the box empty)
Set 'NAT Keep Alive Dest' to: $PROXY
Leaving 'NAT Keep Alive Msg' blank causes a packet to be sent without any message body. A packet without a body is only 60 to 70 bytes. The router keeps ports open based only on what's in the packet header, so the small packet does the job quite well. The small packet does in fact reach the proxy, but it is ignored because it carries no message body.
A good choice for 'NAT Keep Alive Intvl' is 59. This is the number of seconds between sending NAT Keep Alive Packets. Most routers (maybe all routers) remember routes for at least 60 seconds, therefore sending another packet after 59 seconds insures that the SIP port is always open. Many routers remember routes for longer than 60 seconds, often 180 seconds or 300 seconds. In these cases one can set 'NAT Keep Alive Intvl' to 179 or 299 respectively.
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