Well, NTL telephone lines are cable based, which means they are probably digital. I don't see any reason why they shouldn't work with your alarm system though. Maybe you ring them on 1503 and talk to them.
NTL phone lines, NTL cable and BT phone lines (used for both phone calls and ADSL) are all analogue. Digital information can be sent along them by converting the digits to analogue using a 'modulator'. At the receiving. a 'demodulator' is used to convert the analogue signal back to digital. a Modem (a contraction of the words modulator and demodulator) is a modulator and a demodulator in one box or card, so that you can send and receive digital data.
So... whether it's ADSL or cable broadband, it's a digital signal sent in analogue format.
The fact that the lines are analogue doesn't make them inferior in any way. They just carry the digital signal in 'piggyback'.
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