In telephony, DTMF tones are often used to send information down a line, for instance 'Press 0 to speak to an operator'.

Our services support the ability to send such signals, using the numeric keypad in ipDTL and on our SIP Codecs. For other SIP devices and apps, it may be necessary to refer to the manufacturer's instructions.

Out-of-Band DTMF
Most VoIP calls use 'out-of-band' DTMF signalling, meaning that data is sent across the network, instead of an audible tone.  A SIP client will advertise this functionality to another, during the SDP negotiation phase of call setup.  Our SIP service supports this functionality by default.

In-Band DTMF
There are occasions that an audible DTMF tone is preferable, for instance when a radio studio employs a DTMF decoder so that a sports commentator can alert the producer to say a goal in a soccer match.

It's possible to set our SIP service to always transcode out-of-band signals to audible tones for such purposes. To have this functionality enabled for one or more ipDTL or sip.audio Usernames, please raise a support ticket.

Important: At the time of writing (Oct 2025) we are working on a limitation of our in-band DTMF functionality, as it has been reported that the duration of the generated tone is not long enough for some legacy systems to decode.