The only way I could get it to work, was to go to DialPlan Manager and select the entry for the IVR in question.
I then had to click the XML button and add the following line:
<action application="set" data="effective_caller_id_name=ENG#${caller_id_name}"/>
Where ENG is the prefix. Save it and it works. Issue is, if you modify the IVR, this addition will be lost, and you need to add the line in again.
This is on Fusion 4.4.8. I haven't tried/tested on 4.5 yet.
Thanks