Set "Mode" via Business Rule

Set "Mode" via Business Rule

I would like to set the "Mode" via a business rule, but the option does not seem to exist.

A little back story:
We have a HelpDesk phone number setup and when people leave a Voicemail on the hotline it gets emailed into the HelpDesk as an attachment for processing.

I would like to track these as a mode of submission being "Voicemail"
I created the mode, but would like them to be assigned to this mode automatically rather than having technicians change the mode for every individual request received from the voicemail system

                  New to ADSelfService Plus?