If callers to your auto attendant say their key presses are sometimes ignored, or you cannot get through a bank’s phone menu from your VoIP line, the device or PBX is probably sending key tones the wrong way.
Set DTMF on the device to RFC 2833 (sometimes called RTP event or out-of-band). If your device does not support it, SIP INFO is a reliable alternative. Avoid in-band DTMF – it sends the tones as ordinary audio, which does not survive compression on a VoIP call.
- RFC 2833 – preferred: signals each key press as a special event alongside the audio.
- SIP INFO – sends each digit in a SIP message; also reliable, but not supported by every device.
- In-band – unreliable on VoIP; usually the cause of the problem.
On ATAs and IP phones the setting is in the SIP account or advanced audio section; on 3CX and other PBXs it is in the trunk or extension settings. Restart the device after changing it.
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