DiVitas' Mobile Unified Communications product family is marketed as a "mobility management system". What does this mean? In short, it allows you to use a mobile device (specifically a Windows Mobile client or one of a small range of Nokia mobile phones) as a combined desk phone, mobile phone and instant messaging client.
The server runs on Intel hardware, and is based on a Fedora Linux kernel. It's actually a two-trick pony; as well as being SIP-enabled so you can hook it into an existing telephone infrastructure (assuming the latter is also SIP-capable, of course) it also has an in-built Asterisk IP PBX and can thus be used as an autonomous unit by companies that don't already have a corporate PBX. Since the server is sold as a software-only package you'll have to make sure your kit complies with its hardware requirements, but this should be no big deal (it's a Linux kernel after all, not some wacky, esoteric home-grown beast).
Configuration of the server is via a clean, simple browser interface that has a two-level menu system (a set of "top level" subject areas, each of which has a tabbed set of sub-options). The server can hold two software images at once (just in case you need to roll back after an upgrade), and is pretty simple to get up and running. I had a few little firewall niggles (as you'd expect with a voice device, there's a pile of stuff you have to permit on your firewall for RTP and whatnot), and it was pleasing to see that the various diagnostic tools and info screens helped us quickly figure out why things weren't working.
Once the server is up and running, clients are registered against it based on their IMEI (ie physical hardware address); if you wish, the server can deliver configuration information to the clients to minimise the need for manual entry. The idea is that a client device appears to the user as if it's just any old extension on the phone system, and so you'll configure mappings on the server to link direct-dial external numbers to the client's extension number. As I mentioned, the client's a Windows Mobile application, though there's also a flavour that runs on a Nokia E71 (which is what I had) and a Nokia N95. I did ask the obvious question - namely whether you can run it on an Apple iPhone - but the answer was that since the iPhone doesn't currently let third-party applications run in the background, it's not currently a possibility.
So far, so good - it's a phone and an IP PBX. The bit that matters is how the client and server intercommunicate, and the answer is: by whatever means is available and most appropriate (the latter basically means "cheapest"). When you're sat in your Wi-Fi-equipped office, the client will connect over the Wi-Fi network and all calls will take place via the wireless network. If you start walking around, and go outside - or at least outside the workable range of the Wi-Fi connection - the phone will monitor the diminishing signal and will switch to an alternative connection. So the next choice would be a GPRS/2.5G/3G connection via the mobile network (note that it uses the native bearer - that is, it doesn't try to do VoIP over GPRS), and if such a data connection isn't available then it'll drop back to a good old cellular phone call. It's sensible enough to monitor the signal strength as it drops, incidentally - so it'll wake up and switch to the "next best" connection before the current connection becomes too weak, and thus you notice no loss of speech, just a little "beep" to tell you it's switched.
The display on the client is comprehensive and usable, and because each client is basically a computer on your corporate LAN, it lets users see who's away, who's online, and so on. As previously mentioned you can use IM-style messaging, and there's a big connectivity icon that shows what type of connection (WiFi, 3G, etc) is currently in use.
Since the server is basically a full-blown PBX, it can do sensible stuff such as intelligent call routing (so you can tell it, for instance, to route calls to your other mobiles through the GSM network if you have some special corporate deal that makes it cheaper than routing through landlines) and you can of course have multiple units in your various distributed offices and set up routing appropriately between them. If you want to bolt it onto your existing PBX via SIP, that's fine - so you could, for instance, drive your DiVitas clients via the DiVitas server but use your main PBX for voicemail services.
The client's quite nice to use, as you'd expect - it is, after all, just an extension to your mobile phone's built-in software. So on our Nokia E71 the only difference on the homescreen was that the left-hand multi-function key was labelled "DiVitas"; when you made a call, it was handled transparently by the DiVitas client, and routed through the appropriate network.
The server's also nice and friendly. Of course, you'll need some understanding of the various technologies in order to configure them, but this is to be expected on any phone system. And the reporting function's pretty useful, too - you can very easily get summaries of things like how long each handset has used its WiFi connection versus how long it's spent connected to the cellular network. Oh, and the Asterisk PBX configuration has been nicely integrated into DiVitas's own GUI.
All this said, there are some drawbacks. If your WiFi connection suddenly dies because someone unplugged the access point, your call will go quiet (but shouldn't drop) until the software fires up an alternative. And if your chosen cellular network is one that doesn't allow a voice call and a data connection to be live at once (which according to DiVitas is the case with Vodafone, for example) then should your DiVitas client be using a last-resort GSM connection, it'll queue all the packet data (updates of who's on line, IM traffic, etc) until the call terminates. But the main thing that struck us during testing is that since there's a big reliance on WiFi, you have to be darned sure you've planned your wireless LAN correctly; wander to the kitchen to put the kettle on while you chat and you may well find that the walls between you and the WiFi LAN have eaten your wireless signal and the phone's flipped over to a paid-for GPRS or GSM connection.
But we'll forgive DiVitas these drawbacks. After all, if you're dumb enough to lash together a half-baked WLAN and then rely on it for your phone service, you deserve everything you get. And where they can mitigate a problem, they've done so - so if it can't get data through it'll queue it until it can.
The only real criticism I can level, then, is that they need to support far more phones than just the two Nokia models it handles right now. I did actually consider the E71 when I was due for my annual free upgrade from Orange recently, and discounted it because although it's cool, the battery life is rubbish compared to the string of Sony Ericssons I've had for the last few years. So they really do need to get going and make the software work on some of the more mainstream non-Windows mobiles.
Before you take the plunge, remember you'll need a Wi-Fi network that's properly designed and gives good coverage to make the most of the system.