To check the default gateway on the phones, use the configuration menu on the IP phone.
Hold down the Up and Down buttons together and then release just the down button. While still pressing the Up key dial 234 on the dial pad and then release the Up key.
Select Yes for Network Parameters, and Yes to View current values.
If the default gateway is not set to the MBG server, then make sure the DHCP server for the phones has the router option set (003).
Hope this helps