I write as someone who still has the copy of his first personal mobile phone contract with an organisation that was called BT Cellnet. Of course BT decided that mobile wasn't core to its business and sold it off as O2. Now its back as a mobile network operator (MNO) having bid a significant amount (albeit of a lower than expected overall government take) for UK 4G spectrum in the recent auction.
Furthermore, BT paid for what was described as a 'meaningful quantity of spectrum'. It may have been coincidence, but this week BT announced it had bought ESPN's UK and Ireland TV channels, and as I have postulated before football 'on the go' needs 4G.
Irrespective of what BT intends to put across its spectrum, having BT back in the MNO game can only be good for competition.