I have no idea. This is either a defect, or a core configuration change to your environment for how DTMF is negotiated and MTPs inserted. Either way, it's probably outside of the scope of UCCX. I.e., There's not going to be a setting in UCCX you need to change to make this work. It's going to be in CUCM and/or your Voice Gateway. It might be better to post this in the IP Telephony forum.
For starters though, you'll need to determine what DTMF relay mechanism is in play before the transfer attempt, and then what relay mechanism is negotiated after. And whether or not an MTP/XCODER is being inserted to help out with a mismatch of media capabilities.
For your reference, UCCX will only work with call legs which negotiate Out of Band DTMF (such as: H245 alpha, SIP KPML, MGCP, SCCP, etc.). too many times I see people with either their dial-peers on their voice gateway, or the SIP trunk in CUCM to their voice gateway hard coded/limited to RFC2833/RTP-NTE, and they don't realize how that's bad. You should always try to include one OOB option with your inband option, and your SIP trunk should always be No Preference.