When the V300 Line/Phone led flash in alternating fashion, it's telling you it's WAN connection is dead. The WAN light is off too. so internet and VoiP phone will not work
When the V300 Line/Phone led flash in sync, it can't register to the SIP server.
The V300 does not appear to be listed on the Mynetfone website though. Have you checked to see that your V300 has the latest Firmware. LINK
It may be that the V300 is not getting it's IP renewed. Can you alter the IP lease time? Do you have the V300 set up for PPPoE, or do you have it set to static or DHCP (refer CH4, page 25)