28 | Section 2
few times before it really is his turn. Since this sound is played only to callers being switched
to air, it clears the confusion. The Producer/call screener needs to explain this to callers.
DTMF (on air). Choose the DTMF menu to set these tones. Dialling keypresses send call setup
messages digitally to VoIP lines or gateways. Though they are not sent to the Telco line,
standard DTMF (Dual Tone Multifrequency) tones are played on-air - but they are scrambled so
that listeners are not able to easily detect the number being dialled. The correct DTMF tones
are played to the VSet handset and loudspeaker.
Ringtones. Choosing the Ringtones menu lets you choose different ringtones for each incom-
ing line position on the VSet. Ringtones with higher numbers take precedence. For example,
If one line has ringtone #1 assigned to it, and another line has ringtone #5, and both lines are
ringing, the Vset will play ringtone #5.
SIP and DTMF
The VX uses standard SIP procedure for dialing. A SIP call setup message containing the
number is sent to the IP network. The gateway to analog POTS lines generates the DTMF (Dual
Tone MultiFrequency) audio signal that the PSTN uses to direct the call. A gateway to ISDN
lines translates the SIP message to the equivalent ISDN call setup message. The VX does not
generate DTMF audio. Likewise, the gateway translates call progress signals from analog tones
or ISDN messages to SIP. The default is a simple ‘click’.
There is sometimes the need to send DTMF to the PSTN after a call is connected, such as for
automated attendant systems. In this case the VX sends a special SIP message that tells the
gateway to generate the corresponding audio DTMF tones, according to the standard specified
in the IETF (Internet Engineering task force) RFC2833.
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