They didn't "require" a separate line. Yes, a burglar could disable the alarm, but that's possible even with a separate line by simply cutting the line externally!
In 1985 they would not install an alarm that was monitored except by installing an additional phoneline.
This is what existed in 1985:
"Digital Communicator
Also known as digicoms, these systems were among the first digitally monitored systems to be launched.
Digicoms use their own dedicated phone line, which must have incoming calls barred. This is to prevent a cunning intruder calling the line to engage it, so that the system can't call out.
The digicom is mounted in the alarm control panel. When the alarm is activated, it dials out to the ARC and sends packets of data in a secure encrypted format that's decrypted by software at the ARC. The ARC will then take appropriate action by notifying key-holders or calling the police.
Pro's
Alarm activations are securely sent to a 24/7 monitored Alarm Receiving Centre that will always respond
It's relatively cheap to buy the digital communicator and it can be moved with you if you move house
Con's
You have to pay line rental on the dedicated "incoming calls only" phone line You have to pay an annual subscription to the monitoring station (ARC)
You will need an annual maintenance contract with the alarm installation company
If the telephone line is cut, then the system will not be able to call out "
https://securityalarms.co.uk/monitored-alarms/#digital-communicatorIn the 1990s Redcare was created and they are able to use the regular phone line for it. An additional line is not necessary.
"Single Path Signalling
A single path signalling system can dial out using either the GSM mobile network, GPRS mobile data network
or a fixed phone line. It can also send more sophisticated information to the ARC. For example, it might report the type of breach, such as a personal attack or standard alarm activation, including the zone triggered within the property.
The biggest advantage is that the signalling path itself is monitored.
That means that if the phone line is cut or the GSM or GPRS signal fails, the monitoring station will know about it and will treat it as intruder activation. Pro's
Alarm activations are securely sent to a 24/7 monitored ARC that will always respond
The signal path is monitored, so any attempt to tamper with it will be notified to the ARC
It can send out more detailed information than a digital communicator, so keyholders or police are better informed and ready to deal with the situation when they arrive on site
You can use an existing fixed telephone line, with a cable extending it to the alarm panel Con's
You have to pay an annual subscription to the ARC
If installed on an existing line, you may need to add an ADSL filter which separates digital data transfer from voice You need to pay for a monthly SIM card subscription if you choose this communications path"
https://securityalarms.co.uk/monitored-alarms/#single-path-signalling