It works! Took a long time to figure out, but this is how to make a N800 (or Windows machine) connect to the Internet via Bluetooth via an emobile H11T:
The last one is critical: Windows does not do this, so it works. The N800 turns it on by default, so it fails. MacOSX turns it on too (see http://www.taniwha.org.uk/) which is how I found this out. Usually if compression is not supported, it should be negotiated during the PPP handshake. But that’s not how it works in real life.
It took me about 5 calls to the phone support of emobile to find this out. When they just wanted to replace the phone with a new one, the last test was to reset all settings and try again, and then it worked finally.
The issue which made this experience painful is: if you try to connect with compression on, then the phone will not allow any connections any more. To fix this, you need to reset your phone. Which means losing all your data, time setting, network settings, Bluetooth pairings etc.