FXS/ATA unregistering from Asterisk PBX

Hi,

I have a problem with my current configuration. My FXS registration
expires every 60 seconds. This is always registered successfully.

However, when i receive a phone call the extension registration expires
and requires a reboot.

The extension can still make outgoing calls.

I have using an "AT VP-504e FXS (SIP)" with Asterisk@Home. I can
provide a copy of syslog (for both extension and pbx if required)

Any ideas appreciated!

Regards,
Dave