The 176 options (CallSv, FileSv, port) are entered in the same DHCP configuration sequence. You should be able to get an address lease, unless your broadband company is doing something stingy with your DHCP server and only issuing leases to certain MAC addresses. Could it be a layer 1 issue?
I assume that when you configured the phone on the internal network and put it in group 876, that it downloaded the proper firmware (a10bVPN23_2.bin), and you have configured the VPN settings either manually or via the settings files?