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VoIP and DTMF Issues

Updated on January 27, 2011

What is DTMF?

DTMF or Dual Tone Multi Frequency signalling refers to the process of recognizing a particular keypress when the call is in progress. Most of us are familiar with the procedure for IVR systems where you have to press a number in order to choose some option from a menu. Some of us even remember the times when key presses used pulse dialing - every number pressed would be a rapid succession of "clicks."

On most telephones when you press a key, it produces a sound which is sent over the line just like your voice, which is why you hear it. The system at the other end listens for the tone and thus knows what button you've pressed.

As you must have guessed by now, anything which modifies the voice or encodes it like VoIP does is in danger of sending the wrong signal to the other end, thereby mucking up most IVR systems. In order to overcome this, IP phones and VoIP systems use technicalities such as Inband and out-of-band processes.

DTMF Signals
DTMF Signals


DTMF tones are just sounds sent using the regular voice channel. They can even be generated artificially by playing the appropriate tone in real life. Some of us have even heard this in regular conversation even when no button has been pressed. This is because a high pitched voice has the ability to fool the system into thinking that a DTMF tone is being sent.

Since PSTN lines don't compress their audio, they're able to send the DTMF sound perfectly. VoIP however in an attempt to save bandwidth uses codecs that compress the sound and this can result in a wrong tone being sent.

Therefore, IP phones have an option to send the DTMF tone "out of band" meaning that the extra signal isn't sent as part of the conversation but in a separate channel. This overcomes the problem and the signal is sent perfectly. You can select which option to use. Go through some mobile VoIP phone reviews to make sure the phone you're purchasing has the necessary features.

There are two protocols currently in vogue for sending such out of band DTMF signals - the SIP Info protocol and the RTF2833. The latter one is most commonly used and IP phones will allow you to set various options such as which protocol you want to use as well as what the size of the RTP packet will be. Due to inconsistencies with timing etc, the RTF protocol seems to be the one most services will be using including public ones.

Call your hosted business VoIP services provider  and ask them about the optimal settings for the out of band DTMF settings. If you're using RTF2833 and they receive only SIP Info, there will be configuration problems.


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