If you answer the call immediately as you receive the call indication the phone seems to be in a specific call state where answering is not allowed yet.

If you answer the call using the described DBUS message after a delay of e.g. 1 s it works out fine.

Another (probably better) approach is to register to the CallStatus DBUS message ("com.nokia.csd.Call", "/com/nokia/csd/call/1", "com.nokia.csd.Call.Instance.CallStatus") and wait for a call status >= 2 (=CSD_CALL_STATUS_COMING) after the "Coming" message.

Best regards,
Roman