The simple answer to this question is NO. Abacus is by design a bulk call generator. It will respond to incoming call request based on the call status at the time the request (an INVITE message) is received. It is not a SIP protocol tester whereby users can configure and provide the intended response freely.
Having said that, Abacus does support a feature called "Configurable SIP". This feature will allow users to add and change the SIP messages / response. However, the change is very limited and is only at "message" level. What this means is that the internal call and test status is NOT changed as a result of these changes. Furthermore, a change in the message / response will likely have an impact on the recipient of the message / response. For example, if users remove certain mandatory headers from the message / response, the receiving side may not have enough information to further process the call, resulting in a call failure. With that, users must be very conversant with the SIP signaling protocol and is stronger advised to give due consideration before making the change.