There are many reasons contributing to that. Followings are some possibilities and the approaches to resolve it:
1. Abacus is not sending the correct DTMF digits:
This could be due to mis-configuration. Check your configuration to see if there is any problem. If no obvious problem is found, open your audio monitor, and listen to the channel. You should be able to hear the DTMF tone (unless you are using RFC 2833 to send DTMF tone in VoIP signaling). If you are doing VoIP test, capture the RTP media stream and see if the RTP stream has the DTMF sending out correctly. In case of using RFC 2833, you should be able to see the actual messages containing the digits sent.
2. The SUT is not receiving the DTMF digits correctly:
In this cases, you should check with the vendor / developer of the SUT about the capabilities of the SUT. Use appropriate debugging tool as needed.
3. The DTMF signal is distorted when traversing the network:
This can only be confirmed by capturing the signal at the SUT / receiving side. You may need to have a third party capturing device / software. Once you get the capture, you can analyze the signal to see if it is not unduly distorted. Typically if VoIP is used, delay and packet loss could be the sources of the problem. Use of codec other than G711 is also a possible culprit. If the signaling protocol allows, use RFC 2833 could improve the situation.