[otrs] Ticket Creating Customization Help

Hello, I want to create another combobox inside the /opt/otrs/Output/HTML/Standard/AgentTicketPhone.dtl Imagine this, when the user is going to create a ticket he will se the service combo, where will have only the major services, like: "Incident", "Problem", "blabla" and when the user select a major service, a second combobox, is loaded with it subservices, say I choose "Incident" in the first combo, the second will show me all the services that begin with "Incident::" I was thinking on creating a ACL to load the first combo, and ajax to load the second one. Could someone help me with loading the second combo? Thanks