I got an offer in the mail from AT&T for a "3G Microcell". 
(click for larger view) which offers "more bars in your house". It is free, as long as I keep it for 12 months. Normally a $199.95 value, I decided to check it out.
What is it: A device you plug into your home network. Your cell phone sees it as a cell-phone tower and, since it is closer to you than the local cell-tower, uses it for phone calls. The phone calls go out as VoIP through your internet connection. It works with any ISP (I have FiOS, not AT&T.)
How does it work: When you cell phone is near it, instead of saying "AT&T" on the status line it reads, "AT&T M-Cell". Phone calls that start from the cell transfer to a real cell-tower if you leave your house. However, if you start a phone call away from the house, it doesn't transfer to your Microcell.
Setup: Setup was amazingly easy. I plugged it into my LAN, it got an IP address via DHCP and "phoned homed". It upgraded a software upgrade and was working about 60 minutes later. At the AT&T web site you list which phones can use it. I was glad to see that the interface gave me defaults based on which phones I have that are 3G, thus not requiring me to enter the phone numbers manually. I can also add phone numbers manually but so far I haven't needed to.
Geek stuff: I changed its IP address in my DHCP server while it was doing its software upgrade and that didn't create a problem. To be more specific: It booted up in my DHCP "free pool" but when I saw its Ethernet MAC address, I quickly assigned it a static address. During the upgrade process it rebooted itself and came up on the new IP address and continued just like it should.
More geek stuff: It doesn't answer on any TCP ports. No admin interface, no SSH, no nothing. I like that. You connect it, it "phones home" and starts working. That's how a device like this should be. You change settings at the AT&T web site.
How does it work: Great so far! I get 4-5 bars in the house instead of 2-3 bars as I used to.
The purchase process: I took the coupon they mailed me and visited an AT&T Store. They asked for my phone number and handed me the box. It couldn't have been more simple. Actually, I was disappointed that it was this simple. (I was a bit surprised. Shouldn't the salesperson have asked me if I had questions? verified I understood what it was?)
Why is this free? I can only speculate why AT&T is giving away this "normally $199.95" value to users.
- First, if you cancel your AT&T contract in the next 12 months you have to pay a pro-rated share of the $199. Thus, it benefits AT&T in that it discourages you from canceling your contract.
- Second, for an added fee you they'll make all the phone calls that start from your Microcell free (i.e. not count towards your "minutes"). I couldn't find what the price was, and I didn't opt for it. This would be excellent for someone with a home business that is on the phone all day. I just want better "bars". I don't use all my minutes each month.
- Third, it takes the load off their cell towers. I can imagine a person with a home office hogging that cell tower's "slot" all day long. This has got to be comparatively cheaper.
- Lastly, if you have a home office and this lets you eliminate your land-line, it is money out of the mouth of one of AT&T's competitors.
Again, that is all speculation.
I wonder if I got the offer in the mail because everyone gets the offer, or did they do some calculations and figure out that my usage profile makes this cheaper for them. I wonder.
Having phone calls be "free" from the Microcell is an interesting situation. (This is the "extra cost option" I mentioned before). If the call starts on the Microcell then you walk away, the entire call is free. (Someone once told me this is due to the fact that the teleco's billing systems are so inflexible they couldn't implement the feature, so you win.) If you start the call on the normal cell system the call doesn't transfer to your Microcell, so you don't save money that way.
Some negatives:
The box needs line-of-sight to the satellite for GPS. That means this has to be plugged in near a window. Why is this? 3G signally requires very specific timing and the 3G designers decided to require all cell towers to be perfectly in sync via GPS. I wonder if this means they didn't know what NTP was, or was NTP not an option.
The box has to be connected to your ISP via hardwire, not WiFi, connection. I don't think you'd want to trust WiFi to your voice communication, so this is a good design decision. However, it means you'll need to place it near a network jack or near your ISPs router. If that isn't near a window, plan on getting a long cable.
If you start a phone call from a normal tower and then walk within range of the Microcell, the call doesn't transfer to it. That means if signal is really bad in your house, you might have to hang up and redial.
If you receive the offer from AT&T to get this for free, there's little to lose here. If you have a home office and are on your phone all day, this is a big win, especially if you can eliminate a land-line.
More info is available here: www.att.com/3gmicrocell
I got the same offer.
About the GPS requirement. It only seems to need GPS signal on boot. I wanted to put mine in a central location in my home, the room has no windows. I decided to try plugging it in by the window with the power running through an UPS. After all the lights stopped flashing I moved the UPS and the microcell away from the window. It has been working fine for about a week now.
Also, for people who live in rural areas (me). If AT&T can't find your address on Google Maps you can't activate with out calling in and providing them with the GPS coordinates of your house. It will try to auto activate then sms you error code 102 and a phone number to call. I called in and within 30 minutes I was up and running.
Hope that helps.