BT can't seem to kick the mobile habit. Since selling O2 (nee BT Cellnet) to Telefonica back in 2002, the former state-owned telco has been in bed with all of the mobile operators. Now it has signed up with EE.
What makes it interesting this time around is that BT now has spectrum. This makes the big difference between BT being an mobile network operator (MNO) and a mobile virtual network operator (MVNO).
That spectrum is a small slice of 2.6GHz which BT bought in the 2011 4G auctions. BT hasn't enough spectrum to cover the whole country and to have full coverage it would really want 800MHz as well. This is why BT needs EE – which has the adjoining blocks – so that its customers can roam.
The network Three has made a similar deal to give its customers 2G coverage, but over time as Hutchinson 3's network has built out its 3G and now 4G coverage, that has become less important.
But falling back from 4G (LTE) to 3G is much harder than falling back from 3G to 2G was 12 years ago. Not least because no one has figured out how to do voice.
El Reg spoke to SKT - the Korean network which runs 4G voice or VoLTE (Voice over 3G Long Term Evolution) – which said the best way to deal with the problems of handing over voice from LTE to 3G is to build out your LTE network to a level where you don't have to do it.
But this option is not open to BT, which only has 50MHz of spectrum.
BT is remaining tight-lipped on its launch plans but in a financial presentation at the end of last year it claimed the 4G spectrum will "extend Wi-Fi using licensed spectrum for great range, quality of service guarantee for voice and video which integrates seamlessly".
This poses some interesting technical hurdles. If BT wants to have seamless voice and video from its Wi-Fi to LTE that's tough. Signalling hand-off is complex. If it wants to do voice through EE and is asking EE to support VoLTE to 3G fallback, that's something a lot of people are working on but no one can do reliably yet.
Handing off from BT Wi-Fi or BT's 2.6GHz to EE on 2G, 3G or even LTE is a very interesting proposition – with all kinds of billing system implications as well as the significantly non-trivial voice-routing issues.
We asked BT to confirm its plans for its 4G spectrum, and asked how it expected to deliver "seamless voice". The former national telco responded: "We don't have anything to add at this time".
EE unsurprisingly won't comment on what it will be supplying to its new customer. ®