Even if you could, you probably don't want to. Two suggestions ...
1. Add a new field for when the event really happened and then a calculation to take the new date and if nothing there then use date created.
2. Automate. Voice mail can often be sent to e-mail and e-mail can be auto read in as incidents. Self service works 24 x 7. So rather than waiting till Monday, record it live via an e-mail reader or self service. Of course the easiest way is to only allow phone and self service.
Either way your SLA doesn't run off hours but I must assume you have some need for knowing when it was created.
There is the raise date and the create date which are separate attributes. When it was created is one the ticket was actually saved, but you could present the raise date and allow that to be filled in manually if you prefer.
Thanks I think we could work with that