Could you get away with putting a firewall (ethernet) between their CUBE and your network? On the CUBE end, they should only need the ports and protocols necessary to pass SIP and RTP traffic. Most decent commercial-grade firewalls should be able to handle that without too much fuss.
Are they providiner the connectivity between their CUBE and the PSTN/IPPSTN as well, or is that something you're providing at your site (like through another vendor)? In that case, you may want a firewall on both sides of the device (where the internal one prevents things like telnet from their CUBE to your network).
Of course, another solution would be to VLAN their equipment so that it can only talk to the devices that it should, but that wouldn't cover anything but pure connectivity (they could still telnet from device to device).