It's a pity AT&T didn't embrace UMA. There are no geographical limitations and I used my T-Mo UMA phone overseas to make and receive calls, email, data, and SMS/MMS at regular rates when at hotels in London, Paris, and Madrid. It works with any WiFi hotspot and T-Mo actually had a plan that anytime you were on UMA you were not charged minutes for that call. Additionally, UMA will roam both our AND into a WiFi hotspot. No additional hardware other than your wireless router at home is required. No wireless router? Buy one for $50 instead of having to purchase a specific device for $199.
Wi-Fi Calling for Business Service—Not Traditional Voice over IP With Wi-Fi Calling for Business, the UMA-based smartphone device encapsulates GSM voice traffic and data traffic in IPsec and uses Wi-Fi as the transport. The IPsec tunnel goes out over the open Internet to terminate at one of the geographically dispersed security gateways (SGW). The services provided are not Session Initiation Protocol (SIP)-based or H.323-based but rather GSM services over IP (Wi-Fi). Additionally, because this is a carrier-based solution, the network and the device can constantly monitor both the GSM and IP links and seamlessly route calls to whichever path can provide the best coverage.
I am not so big on UMA. Let me explain my situation. I am in a rural area of the county and it is a cell phone wasteland. I have tried all of the carrirers and none have reliable coverage; very spotty out doors, nada in the house. We had Verizon for many years and our MO was to call forward our cells to the house land line about 2 miles up the road on the way home; reverse on the way into town. November 2009 we went with T-moble because of UMA; I bought 2 used UMA hand sets and a T-Moble router on eBay and went with a no contract plan. Set up was quite easy and I thought this was our answer. In use the service was very poor: lots of echo, latency, dropped calls and word skipping. Tried all the thins I read about on the T-Mobile forums as to router settins, tested my cable connection for speed and jitter, nothing made it any better. Spoke to T-Moble tech support; they played with it but it never got any better. My wife hated it and went back to call forwarding her cell. Additionally, T-Mobile does not see to be supporting UMA that well. No new handsets are coming out with the UMA function othe than Blackberrys.
I went to a Chamber of Commerce function in early December and spoke to a AT&T sales manger about my reception problems. He said if I went with AT&T he would give me a Micro Cell free. He said if I did not like it, bring it back within 30 days and he would cancel and refund my money. We signed up just before Christmas. We love it and it works perfectly. I works so well we are about ready to cancel our land line.
tmo will do the same. I was asked if I wanted a microcell (figuring its same as repeater) or wificalling. I use wifi calling on two $500 phones and it had been super poor till just a few days ago. I read somewhere to change my Uverse wifi channel to 11. Man it is super clear. I know UMA is not the same as wifi calling BUT if you call support half of them call it UMA even though its wifi calling. Wifi calling does no have the cell hand-off the way U understand UMA does.