VoIP was designed for the transmission of voice conversations over an IP network. It evolved to sample and reproduce the human voice, matching the quality of existing telephony. It has succeeded in that endeavor, but two other essential aspects of telecommunications frequently use non-voice transmissions — touch tones and faxes.
Pushing touch tones
Technically, the sounds you hear when you press the digits on the keypad of your phone are called DTMF (Dual Tone Multi Frequency) tones. They were designed as two tones sent at the same time (hence the “dual tone” part of their name), a feat that can’t happen in normal human speech. Because of the complex nature of the sound, VoIP has a difficult time reproducing it when the voice portion (media) of the call is compressed.
Unfortunately, touch tones are commonly used during business calls. Every voicemail system says something like, “Press 1 to page this person.” Or you may encounter a complex auto-attendant system that asks you to input your phone number, account number, or extension of the person that you want to speak to. VoIP has found ways to make DTMF tones work, but the solutions vary, depending on whether you’re using compressed or uncompressed transmissions.
I cover the options for DTMF in detail in topic 6.
Faxing over VoIP
The squeals, squalls, and hissing that you hear during a fax transmission definitely don’t fall in the realm of sounds made by a normal human voice. They’re specifically designed to transmit data representing a visual image and, like DTMF tones, provide a challenge for voice-centric VoIP. The VoIP community of engineers and programmers have expended a great deal of effort to establish a viable way for faxes to be sent within a VoIP infrastructure.
You can get a breakdown of your best options in topic 6.
