Here's a great side benefit of having an IP phone in teleworker mode hanging off of a system somewhere out there on the Internet - you have a close-to-instant warning system about Internet connectivity issues.
Take this morning... I walk into my home office and see that one of my phones has come out of its sleep status and the backlight is on and showing "CONNECTION PENDING..." with these black square boxes next to it. I glance at another IP phone: "PLEASE WAIT"
Oh, %#$#?!. It's going to be that kind of Monday morning!
Yes, indeed, as I woke up the PCs, I did indeed have no connectivity. Couldn't get to any websites and all the IM clients were cycling waiting to get connected. After doing the usual power-cycling of the cable modem and verifying that I could get an address but couldn't ping beyond the next hop router, a relatively-quick call to Comcast brought the word that there was a "partial outage" in my area and that connectivity might be going up and down for the next two hours.
Great. Wonderful way for a home office worker to start a Monday.
But it did remind me of one great benefit of having these IP teleworker phones - they are a great way to know almost instantly whether my connection is up. If I'm in the middle of doing something on my PC and it seems like connectivity is down, I just turn my head to look at the phones and can see very quickly if they are up. Likewise, if I'm downstairs using my wife's PC and it seems like Internet access is down, I just go up the stairs and pop my head in the office... first glance is to see if the phones are up.
It's a great side benefit of having the phones, although admittedly it wasn't anything on my mind when we were rolling out the Mitel Teleworker solution back in January 2003. (Full disclosure: I was the product manager for the product when it was released.)
Now, this works in my case because the phones are using Mitel's own MiNET protocol and always have an encrypted MiNET connection established back to the Teleworker server sitting on the edge of the corporate network. If the connection is broken, the phone flags that by displaying the aforementioned warning messages. It's not *instant*, but typically within 30-60 seconds of the connection being down the messages appear. If the phones were, say, in SIP mode connected to a SIP server out there, I wouldn't get the same fast notice because in SIP mode they are essentially stand-alone endpoints - think of them as mini-computers with a phone handset. The first time I'd really notice was when I went to make a connection (or if the phone went to make some regularly scheduled connection and couldn't and put up an error message).
This "side benefit" is, of course, not at all unique to Mitel implementations. Basically any other IP phones that have "always-on" connections back to a central server will have the potential to do the same thing.
It works the other way, too, in letting you know when the connection is back online... while I was on the phone talking to the pleasant customer service rep at Comcast, how did I know that my Internet service was restored (at least for the moment) without looking at my PC? Simple...
... all my IP phones were back in operation.
 And yes, I have several teleworker phones- three to be exact, but hey, I'm working on emerging technology stuff so I have to be able to experiment and work with these phones. They are also on different switches and trial systems.