The NT-IP-G9W includes Intercom and Mobile Phone Calling using our managed SIP server, free phone technical support and even remote management from our secure office if needed.
In my book, that's a back door. Supposedly, their remote solution is unencrypted and has the password "letmein" and it is impossible to turn off or change the password. Bogus alert phrase: "our secure office" -- WTH has their ability to lock their office to do with securing traffic over the Internet?
OTOH, the chinese OEM unit likely has similar problems.
These devices can only be dealt with by compartments, so that they're left to play in a shallow pool where they only can talk to our devices and nothing else. Then, all external connectivity is proxied (no, not NAT'ed, this kind of crap requires a full L7 protocol aware proxy; with NAT it'll call home and set up a reverse proxy in no time) through something we control and regulate.
For SIP signalled multimedia, there is a broad spectrum of software available, like Asterisk or its web frontend package FreePBX (it is an Asterisk inside) that can do this proxying, including acting as a B2BUA (back-to-back user agent, a full proxy between two calls, one internal, one external). Normally (I once built a SIP based audio transport system for broadcast use) I'd stay away from the B2BUA and such, because it can lead to media conversion and delay, but in your situation it is probably the least painful.