According to voip-info.org/wiki/view/Aste … p+dtmfmode
dtmfmode=info does not work with Asterisks voicemail
Is it still true?
My SIP-adapters have rfc2833 implemented in a manner which Asterisk doesn’t like. I’ve tryed to use inband on the phone side and rfc2833 on the SIP trunk side but this does not work - Asterisk in not translating the tones. I’m looking for a solution which will work with local Asterisk VM as well as with provider’s VM.
It seems that I have to answer my own question 
dtmfmode=info does work with Asterisks voicemail, as soon as SIP UA is using appropriate SIP INFO message format.
Old good PAP2 (works fine):
Message Header
Content-Type: application/dtmf-relay
Message body
Signal=1\r\n
Duration=100\r\n
VTech ip8100, PAP2 v2 and their brothers (Asterisk reports 415 Unsupported media type):
Message Header
Content-Type: application/text
Message body
Signal=1\r
Duration=70\r
It will be nice if it will be possible to patch Asterisk to parse those messages in more relaxed manner.