VoIP & Issues with DTMF

DTMF (Dual Tone Multi-frequency) are signals/tones that are sent when you press a telephone's touch keys.  These tones (or data signals) are used to access voicemail (passwords) and navigate IVRs or attendants for large companies like banks.  At times you may find that the far end will not recognize or react correctly to the input you made from your phone.  These problems are typically DTMF issues.

There are several ways these tones are sent and depending on your connection may vary between one or another.  Typically with VoIP DMTF tones are delivered either in-band (as a beep) or out-of-band via SIP or RTP signaling messages.  Some  types of delivery options are:

• Inband-  With Inband digits are passed along just like the rest of your voice as normal audio tones with no special coding or markers using the same codec as your voice does and are generated by your phone. 
• rfc2833-  (Preferred setting in most cases)  Is a standards based way to define signaling for various events including DTMF tones, fax-related tones and country-specific subscriber line tones.
• Info-  SIP signaling will negotiate preferences with the other end to help establish the call's parameters.  The choices can then be set for what to send (outbound) and what want to receive (inbound).  Only available with SIP channels and is transmitted through a SIP message.
• Auto-  Uses rfc2833 by default, but will switch to inband DTMF tones if the remote side does not indicate support of rfc2833.

What are the problems with DTMF and VoIP?

In some VoIP routes a switch may be configured to detect in-band DTMF which is sent by the VoIP ATA, but then switches to an out of band RFC2833 DTMF required for an upstream provider.  This upstream carrier then terminates the call to the PSTN, possibly to a voice mail system, which will require regeneration of the audible inband DTMF tones.  The switch has to detect and remove the tone sent by the ATA from the audio stream because the upstream provider specified RFC2833 DTMF.  At times the switch can't always completely remove the in-band DTMF tone which is a problem, because by the time it has detected the DTMF tone, it has already passed a short amount of it. This small amount of in-band tone along with the RFC2833 tone sent are both received by the far end voice mail system which will then register an error (problem), possibly an invalid mailbox or invalid password.

Troubleshooting DTMF problems with your VoIP connection.

Troubleshooting DTMF issues are hit and miss and may be as simple as using a different DTMF setting and retrying.  But, before making any changes with settings make sure that the issues that you are experiencing are not related to packet loss.  Packet loss can create havoc with VoIP connections and in relation DTMF tones.  Some things that you might look to try are:

• Check for and correct any packet loss.
• Call your provider and ask what options they may have to correct issues with DTMF.  They may be able to change the setting to Inband to see if that helps.

Transmission carried Inband DTMF is only considered to be reliable when the G711 (non-compressed) codec is used.  If G729 is being used and the DTMF is set to use Inband it usually fails due to the compression.   The rfc2833 DTMF setting is generally considered to be the most reliable.