MN500 and VOIP

Hi,
I was getting a VOIP phone service.
I've recieved a box with WAN input jack, LAN Output jack and phone jack.
When I'm connecting the Internet connection to the WAN jack of the device
and then the wireless station to the LAN Output of the device, I'm getting
phne service, but no internet service.
When I'm connecting the wireless before the device, I have both - but the
phone service is interrupted at times.
Checking with the VOIP provider, they say some customers complaint on the
same problem, and solved it with their Wireless station support - since it is
a setting of the station that needs to change.
Does anyone know what is the setting I need to change?

Thanks