Configures the DTMF type for account X.
If it is set to 0 (INBAND), DTMF digits are
transmitted in the voice band.
If it is set to 1 (RFC 2833), DTMF digits are
transmitted by RTP Events compliant to RFC
2833.
If it is set to 2 (SIP INFO), DTMF digits are
transmitted by the SIP INFO messages.
If it is set to 3 (AUTO or SIP INFO), the IP
phone negotiates with the other end to use
INBAND or RFC 2833, if there is no
negotiation, using SIP INFO by default.
X ranges from 1 to 6.