Hello, I'm new to this group :)

I inherited an app that I need to modify. It already successfully manipulates DTMF send duration. I need to also increase/vary the DTMF amplitude.
I added code just below the existing code, and I'm not understanding why SetCallProperty isn't returning success.
Any insight appreciated,

(Existing code)
//Set the Burst ON time:
int SIA_DC05_BurstOnTime_mS = 50,
DIVASDKLib.DivaResultCodes DivaResultCode = CallObj.SetCallProperty(DivaCPT.CPT_VoiceDTMF_SendDuration, SIA_DC05_BurstOnTime_mS);
(returns DIVASDKLib.DivaResultCodes=DivaResultSuccess)

(New code)
//Set DTMF Transmit Level:
int iDtmfTxLevel=2;
DivaResultCode = CallObj.SetCallProperty(DivaCPT.CPT_VoiceDTMF_TxLevelGroup, iDtmfTxLevel);
(returns DIVASDKLib.DivaResultCodes=DivaResultInvalidParameter)