Does this VOIP support DTMF relay which enables us to send a DTMF tone over the channel that is carrying the voice?

Yes, the VOIP has a 'DTMF Out of Band' feature in our current H.323 (H323) firmware platform.
DTMF Out of Band: Y/N
When DTMF Out of Band is enabled DTMF information is encoded at the RTP layer of the voice packet and the MultiVOIP will reproduce the DTMF tones rather than passing them through from the input to the output.

When it is disabled, the DTMF signal is passed to the remote voip the same as voice.

This feature is configured on the Voice/Fax tab of Voice Channel Setup in our GUI configuration software and is enabled by default.