I defined statement below in the coding to send dtmf:
RequestSendDtmf reqSendDTMF = RequestSendDtmf.Create(thisDN, connID1, numberString, null, null);
IMessage reqSDTMFResponse = tServerProtocol.Request(reqSendDTMF);
But I get error 223: Bad parameter passed to function. I've tried to set sip-dtmf-send-rtp = true, but still failed.