An important aspect of Voice over Wi-Fi (VoWFi) is cell phone seamless connectivity over wi-fi networks. T-Mobile USA (part of Deutsche Telekom) has something to say as well: mobile phones don’t have to break the bank, when you can make calls over both cell towers and Wi-Fi routers. T-Mobile claims calls made via cell phone won’t be dropped as you switch between cellular and Wi-Fi networks. Homes with poor cell coverage can still make calls if Wi-Fi access is made available, I say about time!

This is the culmination of the company’s Hotspot @Home initiative, which is going nationwide after several months of trials in the Seattle area. With this service users can make unlimited calls by adding $10 a month, or $20 a month for up to five lines on a family plan, to an existing T-Mobile phone account. The “unlimited” part only includes the VoIP calls made via Wi-Fi, of course – except if the call started on the Wi-Fi side. If the call then roams to the cell network, it remains unmetered (maybe this aspect will change soon).

This may yet be the most important “break-through” since introduction of cellular phones.

I’m sure, in the beginning there will be a small number of phone manufacturers offering dual mode phones, but as this service catches on, I’m sure other providers will jump on the dual mode band wagon.

Users are not limited to Wi-Fi calls on their home networks. The phones will work with any open Wi-Fi connection. T-Mobile will also be selling home wireless routers from Linksys and D-Link that it says are optimized for the service by supporting the WMM standard, part of 802.11e with a proprietary method for setting up a secure link. The routers will be free after a mail-in rebate. T-Mobile apparently won’t guarantee call quality with other routers, nor at non-T-Mobile hotspots. T-Mobile hotspots all have full T-1 lines for backhaul to the Internet, while typical public hotspots with open access are likely only to have DSL or cable connections with no service level agreements (SLA).

The Hotspot @Home network is powered by Unlicensed Mobile Access technology, which enables seamless hand-off from Wi-Fi to cellular and back. This technology has been used by networks overseas by BT in the UK and Orange in France.

The iPhone being a hot product it is, while offering Wi-Fi, will not support this kind of hand-off, as its exclusive carrier, AT&T, doesn’t offer this service. Wi-Fi on the iPhone will likely be used for data only, unless Apple introduces a VoIP application in the future, or people use a third party soft phone software but then I don’t see how seamless switch over will work.