You can use the MBG to VPN to a customers network but this is another trick in your box.
From your office putty into the external address of the MBG
(This has to be allowed in the remote access section of MSL)
Setup tunnels in Putty, see below, copy this and click Add. Make sure to choose “dynamic” and “Auto”
As some of the Mitel kit requires Internet explorer(IE) i will use it as my example.
In IE choose internet options, connections,LAN settings
Now you can web browse to any web site on the customer network, including the 3300, MBG, Nupoint, ETC