Outlook New Mail Persistent Reminder

Hi everybody, 

A customer of ours has a requirement that they respond to all service calls within four hours. As part of this they wish to implement a way that particular mails in the Inbox can be flagged as being urgent and a reminder bell will be triggered every five minutes (to keep the mail very persistent in the minds of the service staff). 

I have checked all the options with flagging but these options seem to just allow you to have reminders at particular times of the day, NOT at periodic intervals. I thought this requirement would be pretty easy to implement, but seems like it's not possible at all? Has anybody ever set up something similar? 

Thanks in advance. 

Jonny.