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Telephone Line Equipment

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BT 150 - CALL SIGN Switch

We have used this handy switch to connect remote time terminals via modems over the public telephone network using the BT additional telephone number piggy-backed onto a lightly used existing phone line like a fax.

British Telecom’s Call Sign Service provides an additional telephone number for your telephone line. This is identified by a distinctive ‘long’ ring when called.

The BT 150 CALL SIGN Switch uses BT's Distinctive
'Call Sign' Ringing to recognise and switch calls.

Now sold out

The BT150 Call Sign switch will recognise the distinctive ringing and route these calls to the Call Sign socket, whilst calls to your main number are routed to Standard socket.

If you are a home worker you can have different numbers for home and business, or in a small office different numbers for phone and fax, without the expense of installation and rental of an additional line.

The BT150 switches the line dependant upon the number called so you can have whatever equipment you want attached phone, answer-phone, fax or modem. It really is foolproof!

Please note: The BT150 switch blocks ADSL modem signals when fitted between the Master BT line socket and the filter. The BT150 should be fitted to the telephone output of an ADSL filter.

Business applications – you can have a different phone number for calls to your fax or computer.

Home applications – if you are blessed with popular teenagers you can provide them with their own number for their friends to call. No more getting up to answer the phone in the middle of your favourite soap or documentary. Live-in Grandparents can have some independence with their own phone.

BT’s Call Sign service costs approximately £6.50 per quarter, or as little as £1.50 if you already have other Select Services such as Call Waiting.


We have now encountered some problems with this device when used in conjunction with a BT line carrying an ADSL-2 internet connection. This only relates to certain fax machines connected to the Call Sign output of the unit and does not happen with former ADSL line signals. It does not affect the use of an alternative telephone connected to the Call Sign output.

What seems to be the case is that the harmonic carrier signal present on internet enabled telephone lines is, in the case of an ADSL-2 signal, able to leek through the ADSL filters and trigger some fax machines or fax modems into answering the call even though a call has been directed to the normal telephone numbered output. The user then hears the shrill fax answering noise and cannot continue a conversation with the caller.

We are currently seeking an ADSL filter of sufficient quality to recommend that will eliminate this problem but so far we have not found one that works well enough.

Use the button to receive further advice.