I went through this too earlier this year. As was mentioned, the line is not changing to "DECT", that is only for your cordless handsets, if you have them already.
The copper wire landlines are being phased out "over the next year", but the deadline has moved several times already. Your future connection will be via VoIP (internet based calls), which some providers will integrate in to their provided router, i.e. BT and Sky, or you need to move to a new VoIP provider and enable your handsets via an adapter to connect.
Either way, you can normally keep your landline number.
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Tony