Call Barring Units 1A and 1B, are line powered and monitor the dialling pulses from loop disconnect equipment, they are not suitable for use with MF4 telephones. Three categories of call barring are available, selected by the customer, using a key operated switch.
When a barred code is dialled or the number of permitted digits exceeded, the unit disconnects the line for ;i seconds causing the release of the exchange equipment.

The categories of call barring available to the customer are:-
 Switch Position         Calls Barred
 VO BAR                  None.
 BAR ISD                 1XX, OOX, 010 and numbers of 11 or more digits.
 BAR ISD/STD             1XX, OXX and numbers of 8 or more digits.
 EMERGENCY               All digits except 9.

The Call Barring Unit 1A is suitable for connection to exchange lines only.
The Call Barring Unit 1B is suitable for connection to PABX extensions and exchange lines. When used on PABX extensions the above barring codes can be prefixed by 9 or 7 by closing switches S4 or S5 respectively. These switches are located on the main circuit board.

Made by TELSPEG Limited

Call Barring Units 1A & 1B are no BT longer portfolio items.
Proprietary items are being supplied by Telspec Ltd.
(Call Barring Units 1A and 1B were available from BT on rental terms only.)
Proprietary units can be installed by any person, but only BT is permitted to make the hardwired connection to the PSTN. All Call Barring Units connected on the network side of the master Line Jack Unit must be maintained by BT.

Approval Number S/2363/3/GI452108.
Approval as supplied for hardwired connection to the BT telephone system.

Call Barring Units 1A and 1B are suitable for connection to hardwired telephones, where the unit replaces the telephone Block Terminal, or in the line before the first socket of Phone Socket installations.
Both Call Barring Units are suitable for installation on DELs. The Call Barring Unit 1B is also suitable for use on PABX and Call Connect System extensions.

Item Description          Item Code
Call Barring Unit 1A      373744
Call Barring Unit 1B      374406

Spare Keys and locks
Replacement keys and locks can be obtained from:-
Messrs Lowe and Fletcher
Bliston Division
Hatton Street
West Midlands

Lock 43Cl3-18, 4 position radial pin lock.
Key for lock. Serial No ....
At September 1989 there is approximately 6 weeks lead-time (delay) between an order being placed and the item being supplied.

With the associated telephone equipment "on-hook" the Call Barring Unit 1 draws a continuous current of 2 micro Amps from the line, this may result in 'Low Loop' line test conditions. The pulse detect circuit inserts a series resistance in the line and with the telephone equipment in use a voltage of 2 volts can be measured between terminals E and 4. The Call Barring Unit requires a minimum line current of 18 ma.

London Re-numbering scheme
It should be noted that calls between the new London 071 and 081 areas will be STD barred when using the BAR ISD/STD facility.


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Last revised: November 29, 2010