It sounds as if the Incident will be created before you call the Service Centre? If so is there any way that you would be able to supply them with your Incident number (either verbally or via email) and then they can send their email in quoting your number. This could then be automatically attached as a note or attachment on your incident using inbound mail.
If not, then I think that the Parent/Child option you describe would work as long as the Service Centre's email they send in generates an Incident as well.
Yes the call is made after the incident is logged, process is student brings laptop to IT Services and then we call and book for repairs. Repair Center do not want to deal with our Incident Number so we cannot use that either.
The Parent child option is what I was thinking about but the main issue I have with that is, can you when you click add parent change the pop-up screen to be filtered so that only the jobs relevant to this would appear or at least put in some custom coloums so that we can identify the incident we need to create the relationship.
A couple of articles which may be of interest to you: