# The Telecommunications (Voice Telephony) Regulations 1997, Paragraph 1: List of facilities referred to in regulation 6(1) and VTD Condition VTD7.1:


DTMF (dual-tone multifrequency operation)
(a) that is to say the fixed public telephone network supports the use of DTMF telephones for signalling to the exchange, using tones as defined in ITU-T Recommendation Q.23 and supports the same tones for end-to-end signalling through the network, both within the United Kingdom and between the United Kingdom and other member States.
Direct dialling-in (or facilities offering equivalent functionality)
(b) that is to say users on a private branch exchange (PBX) or similar private system can be called directly from the fixed public telephone network without intervention by the PBX attendant.
Call forwarding
(c) that is to say incoming calls sent to another destination in the United Kingdom or another member State (for example on no reply, on busy, or unconditionally).
This facility should be provided in accordance with the relevant legislation for the time being in force on data protection and privacy.
Calling-line identification
(d) that is to say the calling party’s number is presented to the called party prior to the call being established.
This facility should be provided in accordance with the relevant legislation for the time being in force on data protection and privacy.

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Source: https://www.legislation.gov.uk/uksi/1997/1886/schedule/4/paragraph/1 (the official version)
Page: https://openlegislation.uk/telecommunications-voice-telephony-regulations-1997/schedule-4/paragraph-1
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.