DTMF - an essential ingredient for Freedom Fone services

Tich and I are recently back from Lilongwe, Malawi's capital city. We were there to run a Freedom Fone user training workshop for six organisations involved in community mobilisation and 2-way information sharing.

We tested 2 mobile networks whilst we were there. tnm malawi worked without any hitches. However, we were disappointed to discover that Airtel Malawi's pre-paid mobile network was unable to transmit DTMF to our Freedom Fone server. This meant that attempts to select an option from Freedom Fone voice menus using the keypad on a wide variety of mobile handsets were unsuccessful.

So what is DTMF and why is it so important?

Here's what Tichafara Siguake, our Tech Support Officer has to share:

DTMF (dual tone multi-frequency) is of particular importance to IVR based services that depend on caller interaction via keypad selections. Without DTMF these services cannot be responsive to the callers' selections. For those who may be wondering what the hell DTMF is, here's a bit of information.

DTMF is the way that telecommunications systems identify the different digits on the keypad. When you press a key on a telecommunications device keypad (for example your mobile phone), two audio frequencies are multiplexed to produce a unique identifying tone for that key. The table below attempts to depict in the form of a matrix, the multiplexing process. If you press digit 1, the frequency L1 and H1 are multiplexed to come up with a frequency representing the digit. The same logic applies to all the other keys.

 Low Frequencies

There are several reasons why DTMF may not be detected by telecommunications systems, these include malfunctioning mobile devices, the signal to noise ratio on a link and we also speculate that mobile operators may actually turn off DTMF reception.

We have come across mobile handsets that do not work consistently with IVR based services; these are usually the low end mobile handsets. When we swap out these handsets with more reputable brands there is a remarkable responsiveness on the same IVR service. In instances where the mobile handset is the problem it is obviously best to change your handset.

Noisy networks can be an impediment to DTMF reception and transmission. Noisy networks are characterised by a high frequency of call dropouts - this may be due to poor network planning on the part of the mobile operator or EMF (electro magnetic frequency) interference. Unless you suspect that the cause of the interference is a nearby source (in which case you should move away from the source) there is little you can do besides approaching the mobile network operator for assistance.

We have also noticed that trying to play Hi-fidelity audio on an IVR service reduces the DTMF sensitivity of a system. It is best to use mono audio sampled at 8000Hz. We believe transmitting high fidelity audio increases noise due to clipping on the channel. Clipping is the process whereby some parts of an audio signal are blocked during transmission.

Another cause of no DTMF responsiveness may be a mismatch in the DTMF signaling method used by the parties in a communication channel. Some systems use in-band signaling and others use out-band signaling. In-band signaling is when the DTMF tones are transmitted in the same channel used for voice. Out-band signaling is when DTMF is transmitted on a separate channel from that used by voice. If the communicating devices are set to use different DTMF signaling, there will be a communication breakdown as both devices are looking for DTMF in the wrong place. Freedom Fone uses in-band signaling. If you can contact someone with the appropriate knowledge at your mobile network operator, find out what DTMF signaling method they're using.

The other reason we proffer as a cause for no DTMF response is that mobile operators may deliberately disable this service at an operator level. This may be a way to control value added service providers who use IVR, who otherwise may not need to liaise with the mobile operators to roll out their services.

Have you experienced problems with DTMF in your country?

Have you managed to resolve issues with the mobile network operator?

We'd like to hear about your experiences.