HP Compaq Presario,Presario 7479 Reference Manual page 84

11-4 Government Approvals
All apparatus connected to this modem, and thereby connected directly or
indirectly to the British Telecom Public Switched Telephone Network, must be
approved apparatus as defined in Section 22 of the British Telecommunications
Act of 1984.
Although this equipment can use either loop disconnect or DTMF signaling, only
the performance of the DTMF signaling is subject to regulatory requirements for
correct operation. It is therefore strongly recommended that the equipment is set to
use DTMF signaling for access to public or private emergency services. DTMF
signaling also provides faster call set up.
The modem may not be used on extension wiring that makes use of plug pins 1
and 6. If in doubt, advice must be obtained from a competent telecommunications
engineer.