The guy who demonstrated the Be There! Personal Multiplexer showed up in his bathrobe (dressing gown!) and gorilla slippers. His thesis: that he was going to be able to pretend to be in the office, because a Data Race server had been installed there, attached to the office phone exchange.
Be There! doesn't come cheap; it's a full server product, and you have to have the right PABX to make it work, too; it has to be Rolm or Nortel or a few models of a few other suppliers. And it looks like overall costs will work out high in Europe, where phone calls are a lot pricier than in the US (local calls are often free here). Installation works out around $600 per user; even here in the US, if you leave the office and travel to another area code, you're going to be spending $30 for a morning's work.
What you do is to dial into the server from your portable (or home-based) PC, using a special phone, which includes a standard 28Kbits/sec modem. At that point, it looks like you're at the office. If a call goes to your desk, the phone back home is the one that rings. If you dial a four-digit extension number, your colleague will answer.
And you have access to the office LAN through the server, and as long as you aren't stupid about trying to download live video, you ought to be able to give a pretty good impression of not being away. No more voice mail phone tag. If the PABX supports it, you can even do conference calls.
The obvious drawbacks are cost, and the fact that it won't work if you have the wrong PABX. That seems to imply a non-US PABX; they appear not to have heard of Ericsson or Nokia, and are not sure about all Mitel exchanges either.