So it's been a few days since we got home and the microcell is still working perfectly. We haven't tried really abusing the upload bandwidth while a call was in progress. The same carve-out that was in place for Vonage on our server is still in place, so that should be sufficient.
I had an opportunity to test the call hand-off this morning. I called the USNO master clock (202-762-1401) in the driveway while the M-Cell logo was on the phone. I stayed on the call (via bluetooth, not with the phone up to my head) until I had driven about a half mile away. The call got a little spotty very briefly as I got a little bit away from the house (mostly because the coverage near our house is not so good), but the call was handed off. While the call was still going, the M-Cell logo stayed in place on the screen, despite the fact that the phone was well out of range and had handed off to the network proper. As soon as I hung up, and the call was ended, the logo reverted to the usual AT&T one. This likely is the phone's way of letting you know that if you have the unlimited calling plan that the call is still free.
When you fire up the Google maps app on the phone and then click the 'location' button, typically what happens is that the phone uses cell tower triangulation to get an approximate fix, then the assisted GPS functionality refines that down to a point. When the phone is in range of the microcell, it's clear that the microcell's GPS receiver has an impact on that initial triangulated fix - the dot starts up being in the right spot with a house-sized circle around it, then when the phone's GPS kicks in, the circle shrinks down to nothing.
One thing that's a little bit of a let-down for a techie guy like me is that the microcell is a bit of a black box. The status lights on the front don't ever change, and so far as I have found, the box has no open TCP listening ports, so it doesn't have an HTTP server or anything of that sort. It's the quintessential Internet appliance in that regard. It would have been slightly more fun had it had some sort of configuration or status UI. Apart from the blinking status indicators while it's booting up and activating, it doesn't do anything visible at all. Since there's no way to configure the box, it's likely that it requires a DHCP server in order to function (this is in contrast to lots of appliances that use DHCP by default, but allow manual overrides with a configuration GUI). This would have been an issue a while ago had I chosen to deploy the box to the outside network instead of behind the NAT, since it used to be that the outside LAN had no DHCP server. That has since changed, but if you have a completely manually configured network (likely a rarity nowadays), you may have some trouble.
It has had no trouble receiving a GPS signal, so far as I can tell, despite being pretty close to the center of the house. There is a window nearby, so it's conceivable that that helps, though probably the bigger factor is that above the unit is just a drywall ceiling, a layer of cellulose insulation, and a plywood and asphalt shingle roof. Wandering around inside with a Garmin tends to work most of the time. The box has a GPS antenna jack on the back, though at the moment there's no documentation about it, nor is there any mention on AT&T's site about an accessory antenna you can buy and situate closer to a window, if required. I assume that when the nationwide roll-out happens they might offer one.
Since the box plugs into AC power, it will die if there's a power failure. The network infrastructure here is on a UPS, but unfortunately it's also located out in the garage. I wanted the microcell a little bit more centrally located than that, since the path from the garage to the office goes through a lot of walls. One workaround I have used before is to cobble together a power-over-Ethernet solution - a pair of specially modified Ethernet cables that break out one pair to pass power. Then just set up one continuous Ethernet path from the back of the garage to the dining room (this is easier than it sounds), plug the power plug from one hack-cable into the microcell, plug the wall-wart into the socket hooked up to the other hack-cable, and then plug the wall-wart into the UPS.